Expedition: general discussions.

2) Honestly, if you can't multiclass with at least one other profession, 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 when leveling up, then even without multiclassing, it should be fine.

If you're creating your own system without linear progression, I had an idea to do something along these lines: the character has two class/specialization points, and choosing them unlocks certain skills to learn (for example, a warrior unlocks weapon mastery, while a mage allows you to learn magic) and gives some passive bonuses. There would be both "basic" classes available to everyone, and some more unusual variations tied to origin or beliefs. For example, if we take the setting of Revan as an example, the "Sword of Dawn" specialization would be an alternative warrior, available only to Anemonians and having its own advantages, at the cost of lacking skills in wearing armor or using other types of weapons. Perhaps, based on the chosen classes, additional bonuses or the ability to upgrade to advanced classes would be unlocked. I don't know if it would be possible to put both points into one class to get some kind of archmage. All of this currently exists only as thoughts on the subject.

Example of what I like (Pathfinder Kingmaker, I don't know any other systems close to D&D): any classes choosing their own magic slots when leveling up, namely inquisitors, bards, priests, etc.

In Pathfinder (and also in the 5th edition, but it's worse there), I also like the concept of archetypes, which allows you to change some class abilities to others, often unique or borrowed from other classes. It helps to tailor the character to the desired concept. Moreover, you can take several archetypes at once (in the computer version, this was unfortunately not implemented), as long as they replace different abilities. The main problem with this was, of course, that some archetypes are frankly weak.

Example of what I don't like: leveling up ordinary warriors, barbarians, etc. is boring. The choice of minor talents is clearly not enough.

In expansions, this is usually what happens. Technically, you can choose anything, but in fact, all the same roles and fighters with low social stats come out, who can only wield a sword.

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.

Well, for example, with more flexible actions in combat. For example, in expansions, we usually have a wargame-wargame, and characters are limited only to movement, using items, attacking, or using some kind of ability that has a specific effect in numbers. There are no other default actions that a character could do (for example, try to push an opponent to another position) or creative uses of abilities with a vague wording (for example, use magic to turn a piece of a stone column into mud and collapse the ceiling).

I also once thought about something like completely free character generation. That is, so that players say what ability (abilities) they want, and the GM determines their strength, limitations, and possible weaknesses numerically, based on ideas about balance and the physics of the world. Then it is entered into the character sheet, and we play. But this seems incredibly difficult to implement.

In my first expansion, there was a similar moment, by the way. After generation, the player could come up with a feature for the character, and I took on the implementation. Honestly, I'm not a fan of this.

In any case, it doesn't make sense to make battles particularly more complicated. Of course, it would be possible to make a system for turning characters like in RI3, for example, but that's it, and not necessarily.

I'm also not a fan of this. Not only does it overcomplicate the system itself, but tactics in battles almost always boiled down to "turn the opponent with one character and one-shot him from the back with another."