Here you go: Nothing!
Isn't it time to mix in some other kind of reward with the endless supply of multi-battle refill packs? They fill in for a "reward" in more and more things every day. I have over 112 of the 20 and 50 in my inbox right now (I got bored counting). That's ... crazy? I mean come up with new rewards if you have to, it's just not even a reward anymore. It's just another thing to scroll past (you can't even sell them!)
if it were me I'd make them sellable for nominal silver (1 per battle in the pack maybe) or, say, come up with additional small ball prizes (mini energy potions of like 18 or 24) etc. Something to mix it up.
The point is that if I cashed in all my refills today I would over 3,000 multibattles available. Compared with all the other resources associated with a day's work in the game, that's nuts.