I mean, good luck. Larian almost certainly isn't going to implement this. Aside from the amount of work it'd require, this thread also shows how unpopular the idea is. Plus, Larian is licensing the rules from WotC - there's likely a limit to how far they can deviate before WotC steps in, and your suggestion is at least close to that line if not over it.
If you want a completely class-less BG3, you'll almost certainly have to rely on modders (or create such a mod yourself).
The closest thing you'll likely get in BG3 is if Larian implements a Lone Wolf mode such that your character simultaneously gains levels in 2 classes, which you then combine with feats (e.g., medium armor master, slasher, extra hp, mobile) for extra customization.
