I love using character builder in D&D Beyond and I love using it to manage my current characters. However, I still have this Google Sheet I created for my self that has a lot of built in logic where I can plan a "build". The reason is that the point of the build isn't the destination, it is journey. It is important to play around with choices at different character levels so that I can see how my build evolves and when it comes online. E.g. when do max my main stat, or when do I get extra attack? This is hard to do with the current character builder because it only ever shows a single point and time. So if you show it at level 20 it is unclear what was happening at level 6.

If you had this functionality I would be giddy. It would be an optimizers dream.

Here is how I actually built it. If developers want a demo of my Google Sheet I would consider posting a video tour of an example build.

1. Page 1. For each build you select a race, a background and set of class/subclasses. I assign each class/subclass a letter that in the progression planner I can select from a drop down. I currently allow 4 different subclasses per build. On this "selection screen" it shows all of the implications, e.g. multiclass requirements, proficiency options etc.
   • e.g. "A" is "Arcana Cleric", "B" is "Rogue/Swashbuckler"
2. Page 2. The progression planner (all the notes below all happen here.)
3. Imagine a spreadsheet that has "character level" down the rows and has columns that progress with the build.
4. The first row is "level 0" and it allows the initial ability score selection and stuff you get with your race and starting class selection.
   • You select your initial ability scores on this row and it uses conditional formatting to highlight when you are under/over a point buy but allows whatever you want
   • It automatically applies racial bonuses
   • I have an area where I can type things like "CHA+2 INT-1" and I wrote a formula that parses that and applies it for you
5. At each level you can select what armor you are wearing and if you are using a shield so that it can do AC calculations for (my formulas include racial and subclass features and it picks the best armor class. Mage Armor is an selection as well)
6. At each level, the Ability Scores copy down, so you can see what your Ability scores will be as you progress
7. You can make manual adjustments to AC and Ability Scores at each level. Same formula as in #3, I just type in "CHA+2" or "Feat Crusher CON+1" and it parses out the ability bump and applies it. You can also type in "AC+1 for magic armor" in the same fiend and it will find that as well.  As above you can apply minuses but this is usually only relevant at level 0 if you are doing a custom race.
8. At each level it has a section where it shows all relevant racial or class/subclass "Features". I just merge it together into a single cell. So if you select for Character Level 3 if your Aasimar takes Paladin 2, you might see "Necrotic Shroud, Fighting Style, Spellcasting, Divine Smite"
9. For selections, like fighting style, I just have an area where  I can type in my selection. I put it in the same column as #3 and #6 so I could type "Defense AC+1" and the formula will pick up the AC bump
10. I have column where I just have key progression notes, so I can keep track of why I'm making the choice. E.g. for my conquest paladin build, I might say "hexblade for SAD" or "Aasimar for fear synergy" or "Wrathful Smite is now the go to for fear effect"
    • I then show the spell slots by spell level (because that formula is a %@%$% for complex multiclasses) as well as the "max spell level" at that row. So I can see that at Character Level 10 I might have 4th level spell slots but my max spell level for paladin is still level 2.
11. I also have many class/subclass specific columns that show progression that show or hide only if needed. This makes it easy to see that if I take another level in bard I get better Inspriation dice, but if I instead take it Rogue, I get better Sneak Attack dice. Here are all the ones I currently track:
    • Brutal Critical
    • Extra Attack
    • Sorcery Points
    • Bardic Inspiration
    • Symbiote Hit Points
    • Lay on Hands
    • Sneak Attack
    • Channel Divinity
    • Infusions
    • Known Infused Items
    • Cantrips Known (relevant to the class at that level, so you can keep track of how many Wizard Cantrips you have vs. Druid.)
    • Maneuvers
    • Superiority Die
    • Blood Maledict
    • Mutagen
    • Hemocraft Die
    • Ki
    • Rages
    • Rage Damage
    • Wild Shape CR
    • Pact Slot/Level
    • Invocations
    • Spells Prepared
    • Spells Known

I'm sure I have more features in there that I just forgot to mention. It would be so easy for y'all to build this since you have all the data as well. Again, if there is interest I can post a video, or screen shots.

Edit: If it wasn't clear, my current Google Sheet does all this, mostly with formulas and look up tables. It is a lot to maintain and I'm not willing to share it because the "save/restore" feature is a little clunky and also I'm sure this stuff is probably proprietary and I don't want to step on any copyright toes.

Edit2: Yes I am a developer and there is some scripting, but most if it is lookups and formula.s

That seems like a lot of effort. Personally I'll just click copy and level up the copy, repeating (and renaming) as I need for plans at each level. Seems simpler.

Of course it is simpler. This is not for you. This is for optimizers that want to plan out a 20 level build. There is a market, but it isn't the core character sheet... that wouldn't change. 
edit: Using your technique, if you want to change your mind and switch your level 2 choice to be a Hexblade dip, you now have 19 character sheets to update. If it isn't clear, when I change a subclass selection on my spreadsheet it auto updates the whole build.

And if I could build it by myself as a hobby... it wouldn't be that hard. The hard part is just the conceptual part.

edit 2: as an optimizer, I have dozens of builds going playing with different ideas. To have 20 character sheets for each of those would be a mess and there would be now way to just sit back and look at the progression. If you open level 20 you would not be able to see the key decisions points or the by level notes.