I actually find mages to be quite fun to play as. Maybe you didn't have the right apparel or perks to make it work? I'm currently lvl 40 and am a complete mage. I haven't leveled anything else at all and I'm doing fine. You usually should research your enemy by going to a library and reading about them to find their weaknesses, that helped me a lot. But your other ideas on thieves and warriors, I haven't played as them yet so I can't really say anything about that. What I will say though, is that if you are willing to make such a mod, it'd probably be in your best interest to create separate mods i.e. a mod for only warrior, a mod for mages and warriors, a mod for all of them, etc.