Ah, that makes more sense, actually.

How would we start that, then?

I'll just post as the rogue in a supposed situation and we'll just improv it from there. We'll also try all 3 routes to learning, so he'll learn from first a party member, then a private tutor, then from an academy.

Scarra, the halfling rogue of your party, is enjoying a well-earned rest as she stretches out on a disproportionately large chair in the middle of the town pub. You've just gotten back from a difficult mission, and everyone kind of went their separate ways to decompress after such a perilous journey. You've all got coin to spend, though as usual, Scarra gave almost all of her bounty away to the local charities, more interested in the thrill of out-thinking those she steals from than in the treasure itself.
She notices your approach and raises her little teacup of some kind of hot, steaming spirits.
"Evenin', sweetie. You look like you've got something on your mind. Come, sit down. I was getting a might lonely anyway." she says. Though the size of a child, she has a matronly way of speaking to the others of the party, especially the hulking goliath that is the party's muscle who is also the youngest.

"Well, there is, actually," The Forest Gnome says as she pulls a chair and sits next to the halfling, "something on my mind." Sorci (her actual name is Sunkirilethanpum, but that's too hard to pronounce, so said Goliath just called her by shortening her profession from Sorcerer to Sorci, and that name stuck) thinks how to address the matter for a moment and then continues.
"Do you remember when we both stood on that carriage - the one that toppled over? We both jumped off, but you landed so gracefully, while I fell flat on my face. It wasn't the only time, though. You always seem so... fluid. When someone tries to catch you, you just slip away. Meanwhile, I'm always too slow to react to those things. I figured... since we have some time... perhaps you could teach me? You know I'm a fast learner, so it shouldn't take too much of your time."
Sorci looks at the halfling with pleading eyes. There was another reason she wanted to learn from her. Scarra might not have noticed, but a particular Goliath always expressed how impressed he is by her acrobatic abilities. Sorci wanted to impress him too.

((I'm simultaneously introducing a possible skill to include in our campaign, a return of the 'tumble' skill, which rogues and other agile classes are SORELY missing from 3.5))

Scarra pats Sorci on the back.
"I can show you what I know, at least. From what I've seen, it's not so much that you're slow and clumsy, but you stiffen up. You know what my secret is? Relaxation. If you're too rigid, when the time comes that you need to move, you won't be able to because your body is like a bowstring pulled too tight. It takes great effort just to move, while a loose bowstring flops about easily." she says.
The halfling leans back in her chair, precariously balanced.
"Push me over. Watch what happens."

Sorci takes everything in and stares for a moment before realising that Scarra is actually waiting for her to shove her. "Are... are you sure?" She asks, but seeing the Halfling clearly anticipating her attempt, she shrugs. "Oh well."
She leans towards Scarra and puts her hands on the Halfling's shoulders, without applying any force yet. Just as she applies all her weight and strength, she wonders, albeit a bit too late, what will happen to her now if Scarra just slips away. What will her weight be applied to?

Athletics for Shove: 4.

Quote from KinaJardine >>

((I'm simultaneously introducing a possible skill to include in our campaign, a return of the 'tumble' skill, which rogues and other agile classes are SORELY missing from 3.5))

 ooc: Isn't 'Tumble' another word for 'Acrobatics'?

I ran my first game using the Cypher System rules yesterday, set in Ptolus. VERY different from 5e- but the players caught on really quick. The first time I played D&D was with the red basic set, then went to AD&D. I skipped over all the versions in between those and 5e (what was I thinking?!) so I never got to play 3.5. How different was it to 5e?

It’s simpler and more streamlined, slowing players and DM to focus on playing. At least that’s my take. I like it better.

((I like how accessible 5e is, 3.5 was pretty rule-heavy. It's the system that was in place when Neverwinter Nights and Baldur's Gate dropped for the PC circa 2001 or so... but the 'Tumble' skill was basically a passive defense that allowed agile characters to completely avoid attacks of opportunity and assisted in dexterity saving throws.))

Scarra simply allows herself to fall. She makes a little 'oof' when she hits the floor, but most importantly, she keeps her drink from spilling despite tipping over.
"Didn't even hurt." Scarra says, looking up at Sorci.
"I just stayed nice and relaxed and focused on my drink." she says, then throws her legs over her head and rolls back up without spilling a drop.
"Ironically, the first step to being graceful on your feet is often learning how to fall on your face without hurting yourself."

Sorci tries to understand, but it seems too strange. "And how would I do that?" she asks. She looks at Scarra's drink. Sorci wasn't incredibly clumsy, but it wasn't unusual if she'd just lose grip of something because she bumped into something or someone. Holding anything that perfectly still looked... beyond her. She wondered if she's asking Scarra to teach her the impossible. "Do I just fall over all day long until it stops hurting?" She thinks about the way she trained with her magical powers, simply releasing them uncontrollably until she found patterns that she can use to tame them.

I see. In that case make a custom feat that gives proficiency with DEX saves and allows the character to avoid AOO, with some limitations, perhaps on the number of attacks to avoid or perhaps an opposed DEX check against the attack, some sort of limitations seem applicable since the ability to get into and out of combat range without having to worry about ‘return fire’ would be a very powerful tool.

You can be a Rogue (proficient with Dex saves) and get Evasion at level 7. 

((That seems to be an awfully high level requirement for a fairly underwhelming feat... but then, a LOT of feats in 5e are kind of underwhelming while others are so overpowered that there's no reason NOT to get them.))

Scarra laughs lightly and waggles her hand in a half-hearted gesture of affirmation.
"Somewhat. The goal is to get comfortable with the idea that staying relaxed even though you know an impact is coming. It's the first step to any acrobatics, accepting the reality that you WILL fall. The grace part comes from recovering. That's what makes it look totally intentional when I tuck and roll out of the way of danger, when the reality is that I just let myself fall away like a ragdoll, then quickly recover. You have to fight your instinct to tighten up and brace yourself for impact. THAT'S how you get hurt." Scarra explains.
She gestures with her head.
"I think I've had enough of my drink or now, anyway. No time like the present to practice! Come on, let's see how you fast a learner you are."

Quote from SrDeebs >>

You can be a Rogue (proficient with Dex saves) and get Evasion at level 7. 

 That’s not a feat. It’s just something Rogues get.

Taking no damage from a dragon’s breath weapon or a level (inset level here) fireball on a successful save? Or half damage on a failed save is dope when you need it.

(Monks get this ability at some level as well. Don't remember which one at the moment, but not too far from Rogues. Maybe at level 7 as well.)

Sorci follows Scarra, sucking her words like a sponge. Relax. Don't be tense. She windered what kind of training Scarra is planning. The fact that Scarra doesn't take time to consider the matter worries Sorci, but she doesn't say anything.

Scarra meets with Sorci in the inn's bathing house. It's a large public bath suitable for up to thirty guests at a time, made almost entirely out of stone and heated and cleaned through some form of magic, and also about half a meter at its deepest... not much of a cushion... and Scarra hasn't activated the runes to heat the water, so it's quite chilly. Nobody is there in the bath house with them.
"This will do nicely." she says, standing in front of Sorci.
"If you can stay relaxed when I push you into the water, despite you knowing how cold it's going to be, you'll have taken your first step towards learning proper acrobatics. What I want you to do is just open your arms when you fall and try and hit the water as flat as you can. If you don't allow yourself to fall and clench up, you're going to take some bumps hitting the bottom of the tub. Fall correctly, and you shouldn't hit it at all. I'm quite serious when I say that the first lesson anyone who has learned to move as I do has started out by learning how to fall without getting hurt." she says.

"Ready?" she asks.

i cannot stop thinking that you just literally doubled up the characters HP for sake of them never dieing !

"But, if I fall flat on the water, it'll hurt too!" Sorci protests, but spreads her arms wide. She breathes in, then out. She's tense. 'Relax', she thinks, and tries to do so. "I'm ready." She says, despite not feeling ready.

Time to roll I guess? Relaxation (Acrobatics): 16.

((Here's the mechanic I thought up for how to roll for personal tutoring))

Scarra's total acrobatics bonus: +6. (3 proficiency, +3 DEX)
Scarra's teaching roll: 17

Scarra's teaching ability (which she rolled poorly for) reduces the number from 25 (since learning a new skill from scratch would be considered 'very hard') for the 'learning curve' since she's acting as Sorci's personal tutor.
The more skilled the tutor, the greater the chance that the practice will be fruitful, but your tutor must be at least 2 proficiency points more skilled than you to teach you(which means that party members, who are normally about the same experience level, likely can only be a tutor for skills you have no proficiency in, meaning it's extremely limited in exploitability and more or less just gives the party skill redundancies if they all just sit around teaching each other from the skills the party collectively knows)

Learning curve: 17. This basically is like hitting over AC.

Roll with DEX AND INT bonuses to try and 'hit' 17. Should you succeed, however much you beat the learning curve by will get you 'skill points'.
The idea is to get enough skill points to buy a +1 proficiency in your new skill. I'm setting the bar at 30 skill points for +1, another 30 for +2 (so a combined 60 points to get a level 1 proficiency), 60 for +3, and so on. These are VERY rough numbers and we'll see how things go as to whether those numbers should be adjusted.