An idea i had while playing was to give a buff for characters that specialize in any power set. Making it more powerful the more powers you select from that set. By set i mean, Force, or Marital Arts or Gadgeteering and so on:

3 powers in 1 set: minor affinity. Buff: 1% extra crit & dmg bonus when using said set.
5 powers in 1 set: affinity. Buff: 3% extra crit & dmg bonus when using said set.
8 powers in 1 set: major affinity. Buff: 5% crit & dmg bonus when using said set.
11 powers in 1 set: Cosmic affinity. Buff: 5% crit & dmg bonus, as well as 10% dmg reduction from said set.

Also you can only have 1 affinity on at any time and the buff only applies after the first use (so if you dont have the buff the first power you use isnt affected by it, only following powers) of that power with an 8sec expiration which is refreshed if further powers from that set are used. This does not include energy builders. Using another power set will also terminate the buff. for example, getting the affinity for fire, but then using a might attack would debuff you.

This is only a suggestion so the figures and buff status is up in the air. I think however something is needed to give a bonus to players that stay in their power sets. Maybe even another smaller buff for staying in power groups (ie gadget, archery and munitions) but those shouldn't be as powerful as set buffs.