Anyway, and I'm not referring to your post in particular, one should consider the basics of OS combat system before looking at other systems/games for inspiration. I'm my opinion its not a system particularly suited for buff specialists, nor one that needs a summoning specialists, considering that filling the battlefield with elementals is one of the simpler way to steamroll through the game, even at the highest difficulty setting.
I'd tend to disagree, as buffs were very, very important and quite overpowered in DOS1. I played with 2 characters only ( both lone wolves ) so having to play with 4 would be new but I reckon there COULD be a place for a buff/debuff specialist whose main job would be to empower his friends, leaving them to focus on other aspects of the game. Actually that would even make combat more dynamic and flexible ( at the expense of strategy maybe ) as you wouldn't be forced to stop whatever you were doing with your big melee juggernaut in order to re-cast his defensive buffs or whatever.
Also, Bards in Rift ( a subclass of the Rogue archetype ). They were quite fun to play, and invaluable in a group, while retaining what makes a Bard a bard : a singer and musician
Simplifying:
1) D:OS is a game with a relatively high killing-ratio. Meaning characters kill and get killed with few hits, and consequentially buffs have to be really, REALLY impactful to make a valid alternative to direct attacks (otherwise the buffer would feel underpowered).
2) All magic schools already have buffs/debuffs among their skills.
3) Offensive skills tend to have long range and/or a wide area of effect, while characters tend to have an high-mobility, so keeping you buffer in the back row would be hard at best, impossible in most cases.
These three reasons combined make me think that a buff specialist would be a waste of time in this system. Maybe a skill tree that combine buffs and melee skills, or buffs and debuffs, could work, though.
