A really good rules system would make all 6 ability scores useful and none of them truly necessary.
It is of course very acceptable that e.g. a Wizard must be at least somewhat intelligent. What is lame and boring, is a game system that makes it always unquestionably better to bump Wizard's Int a little more, than invest in another score, or that it makes certain scores practically irrelevant for some classes.
Therefore, I am not looking for a game where you can be an effective Wizard with Int 3... but I am certainly looking for a game where
there is a choice between being e.g. Int 16 & Dex 10 or being Int 13 & Dex 13 or even Int 10 & Dex 16, and that choice is not trivial. What I mean is that, it is OK for min-maxing to be an option, but the game is really better when min-maxing doesn't give an edge over choosing a more balanced distribution.
5e is already much better than 3e in this regard, simply because of the absence of that minimum spellcasting stat requirement for casting spells in the first place.
I'd like to understand why dabbling should be made easy. It's a class-based engine for a class-based game, which has been class-based since its very beginning. I know the game is yours and you should always play what you like, but there are better systems out there that work with "building blocks" instead of archetype-based characters, D&D is not one of those.
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For myself, I'd love if we return to 2E also in this subject, making multiclass and dual class different things, but neither of those treats classes as building blocks, because with the sole exception of 3E, that's not what they used to be, and they work terribly when used that way.
I certainly want a game that strongly supports each class up to the last level, so that every single-classed PC are totally worthwhile.
For me multiclassing is secondary. As you say, D&D is primarily a class-based game, and multiclassing is an option for at least intermediate gamers. If (some) multiclassed combinations always end up being better than single classes, for me the game designers have failed a major target.