In my opinion:
highest prioroty should be to not alienate non-DLC buyers and thus split the community even more (I'm looking at you, gog-steam-incompatibility). Second to that there should be rewards and lots of it. However, they should be cosmetic, like tons of extra colors for units, etc OR the DLC should be non-mandatory so that DLC buyers+early buyers can play with people who didn't buy either. The Relic games are prime examples for that, where you can play as Dark Eldar or Tau even if your enemy hasn't bought them. This seems to be the fairest DLC model: new factions for early buyers and DLC customers and if somebody doesn't want to buy DLC, they can still play with the DLC buyer.