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1) I think the answer is obvious. The second option is preferable if implemented harmoniously, interestingly, and with a reasonable balance. Since fulfilling all three points is quite labor-intensive, linear progression as a substitute is also acceptable, and it's better than a poorly implemented "ideal" system.
2) Honestly, if you can't multiclass with at least one other class, then I'm more likely to be against it. Because often, the concept is more complex than what's available in the list of classes. But again, if there's enough variability within a class during leveling, then even without multiclassing, it should be fine.
Example of what I like (Pathfinder Kingmaker, I don't know other games similar to D&D): any classes that choose their magic slots when leveling up, namely inquisitors, bards, clerics, etc.
Example of what I don't like: leveling up ordinary warriors, barbarians, etc. is, of course, boring. The choice of minor talents is clearly not enough.
3) EVEN MORE COMPLEX?
Seriously, some kind of flexible system where you can estimate the result of any action in your head based on the character's stats/skills would be cool.
I also once thought about something like completely free character generation. That is, players would say what ability (or abilities) they want, and the GM would determine their strength, limitations, and possible weaknesses numerically, based on ideas about balance and the physics of the world. Then it would be recorded in the character sheet, and we would play. But this seems incredibly difficult to implement.
In any case, there's no point in making battles particularly more complex. Of course, you could make a system for rotating characters like in Heroes of Might and Magic III, for example, but that's just one possibility, and it's not even certain. Otherwise, I don't see any areas that deserve in-depth development.