I'm curious about 2 things, first, barbarians, they can't use armor, but hey have a lot of traits that raise their AC. And spells? How would you manage known spells from memorized spells? And how could you make the player be able to change them like in D&D?
Some of that is a bit outside the scope of the mod, but as long as you're playing GM mode, the GM could always give you a hand with spell management. Also, DOS 2 does have a memorization system that limits the amount of memorized skills you can have active at one time, but I don't think it limits the amount of skills you can know overall. Not quite the same, I know, but I wouldn't have a clue on how to alter it.
I'm not entirely sure if passive skills are possible, but there is probably a way to give physical resistance via a skill and/or temporary armor similar to fortify that would be 0 AP, have a long duration, and be useable once per encounter. In GM mode, the GM could always give the character "armor" and resistances as they see fit as well. That might actually be the best solution.
To get the most authentic feeling D&D game in DOS2 is probably going to take several mods working in conjunction as well as a hands on GM to help simulate certain aspects though. It won't ever be 100% but, with the right mods, it can be close enough for most I think.
