Haven't seen anyone post this yet but thought it could be a nice addition to add raid set piece bonuses. This would then give players some more choices to make as to how they gear their characters, rather than the 3 raid piece + 1 exotic, and a reason to obtain the full raid set..
For example:
2 pieces - Precision kills have a chance to generate 1 orb of light
4 pieces - Expect to find more ammo for your heavy weapon
Anyway, those were just two quick ideas. If you think raid set bonuses would be cool throw your ideas in and I'll try to edit the post to keep up.