Hello all, FJ here!

As stated in the title, I wanted to create a way for Arcane Trickster Rogue's to use their spells with their Sneak Attack, since they have them and there is a disconnect with the abilities I had to give it a shot. Here is what I came up with so far:

Piercing Magic: At 9th level, whenever you cast a spell using an available spell slot or cantrip you know that requires an attack roll, you can add Sneak Attack damage to the attack, as long as the other parameters of Sneak Attack are met.

But what do you all think? I am open to suggestions and thanks in advance

So it would include cantrips? If so, Firebolt or Ray of Frost would probably outclass every melee or ranged weapon attack in most situations. At lvl 9, base damage for those would already be far superior to a sword or bow. It might be okay if you changed it so magic only got half the sneak attack dice at a given level, so it buffed spells while still giving players a reason to use their weapons.

Plus, Arcane Tricksters do get something like a sneak attack bonus at lvl 9 with Magical Ambush, which is pretty powerful with Fireball. Were you thinking of taking Piercing Magic instead of Magical Ambush?

The stacking of arcane trickster and wizard would result in a 11th level wizard with a few more first and second level spells. The feature itself is good, but because of the trickster's limited options, it may be better as a feat for spellcaster. However, Maran is right, cantrips would get out of hand quite quickly. 

An important thing to keep in mind is that the spells available to an arcane trickster are pointed towards gaining advantage on everyday attack rolls, with shadow blade being chief among them.

Well, going strictly by what's written right now, you'd have to use a spell slot to get the sneak attack, and cantrips don't use spell slots, so it wouldn't use cantrips.  As for non-cantrip spells?  Hmm...  there's scorching ray, but honestly?  There's a handful of wizard spells that call for an attack roll, and arcane tricksters don't get a lot of spell slots.  Or options for non-illusion/enchantment spells; at level 8, you only have one such spell choice.   You'll also only have two level 2 spell slots, which is already competing with things like mirror image and invisibility.  Things that let an arcane trickster do, well, magic trickery.  In time, the number of slots will grow, but its still taking up a valuable resource.  Its passable as a nova option, but the few times it comes up per day means its ultimately not that large of a deal.  Personally speaking, I'd much rather be able to cast something like Hold Person with Disadvantage, but that's me.

Even if we did have the whole cantrip thing going on?  At level 9, that's 2d10 damage at best, comparable to the 1d10+dex damge from a bow.... if the bow wasn't magical.  By level 11, the firebolt will be doing 3d10 damage; compare to 1d10+dex+X from a magic longbow.  Now, that looks like more damage, but you also have to keep in mind that we're talking about additional accuracy from the bow as well, and maybe some extra benefits we haven't considered yet, depending on the item.   At level 17, firebolt pulls a bit ahead, unless we're talking about a flametongue longbow with special ammunition, which is entirely possible at level 17.  Or the Arcane Trickster took Haste for a second bow shot.

Compare to using Booming Blade and Green Flame Blade; these cantrips, while melee, stack with other options while increasing damage output.  At level 11, we're talking about 1d8+dex+magic item bonus+2d8 GFB, for a grand total of 3d8+dex+MIB, which puts the 3d10 from firebolt to shame.  Granted, this is melee versus ranged, but the point remains - the damage is higher and you get magic item benefits.

The current ability being proposed seems to be weaker, imho, than the current one.  I fail to see any issues in terms of balance if someone really wanted to go this route.  The biggest difficulty, it seems, is the limited options that the arcane trickster has for getting the evocations (or necromancy) spells necessary for the tactic.  You get a grand total of 4 cantrips and 3 non-illusion/enchantment spells, and lots of competition for said spells.

Magical Ambush gives a target disadvantage on their saving throw if the Rogue is hidden from them. It works nicely with the Arcane Trickster having access to Illusion/Enchantment spells but it's not really offensive (unless you happen to take a spell like Fireball). I want to keep Magical Ambush but have an option for someone who wants to offensively use their spells as a Rogue rather than just a Trickster

I do agree, they at most have access to about 4 spells outside of Enchantment/Illusion over the course of 20 levels but I know most would want the chance to take at least one offensive spell  and I was wanting to find a way to incorporate that into the Rogue's already potent abilities (Sneak Attack primarily)

I do see the cantrips getting out of hand fast, that's why I wanted to avoid them. But I needed input to see if there was a happy medium I could reach or just scrap the project entirely

I'm not saying it's bad.  Rather, I'm saying that you are being too conservative with it.  Go ahead and do the cantrip at level 3, if you want - it seems strong, but it's not really.  By level 9, you are preparing for the next tier of gameplay, so the extra damage isn't out of bounds either.  

You asked about relatively balanced, and I'm saying that the abilities are actually below the curve, if anything.  You are giving up a lot for little in return.  

Ok, I see what you were saying in your earlier post now. So expand it to encompass more so the Arcane Trickster can take a bit more advantage on it. I'll run it by a friend or two then edit the post to show for it. Thanks for the input!