I posted some neat theme ideas in section 4 of my [2.7.0] List of Follower Ideas thread. The ideas include:
- All 3 followers active in single player (1 per player in groups)
- Asheara’s Vestments 4-piece bonus active on all players
- All 3 legendary token powers active on follower (not much of a buff though)
- Legendary gems in amulet slot emanate from follower (4th gem for single player)
See my thread for more details on the pros and cons of each idea.