First one is mine - simply let the main hero from the Saga transfer to Factions. Either as the team member (could be a bit imbalanced though) or coach (his stats affect the team - as minor buff, perk (archers have miss chance reduced by 0,1% per hero's level), 1 renown bonus or visuals). With some fine tuning it wouldn't give the campaign players advantage, but would encourage them to continue the Saga fun in Factions. It would also make the Faction only players want to get into Saga (if they wouldn't already).
Second comes from a friend, who I used to play World of Warcraft with. He is now hooked into World of Tanks (yeah...) and told me that the major thing that got him playing the game was that, apart of tank evolution and customization, the crew is gaining experience. Very little each battle, taking dozens and hundreds of games to level up. Upon leveling they gain a perk of the player's choice, that grants some minor bonus. The bonus is low enough not to make any big difference, but it IS there. And players want it. They want it bad. So bad they spend a lot of time sitting in their tanks for 10 minutes to take one shot, reveal their position and get shot (thats how the game looked like from my perspective - so boring!).
Now the perks can't ever get to a level they disturb the balance. It could be either some miss chance reduction, increased chance to hit after run through, reduced chance to miss the last bloody flail hit, eye color change, custom weapon or the last skin unlock.
What do You think?