Maybe coupons for dndclassics.com.
Thaumaturge shows his usual genius.
I think at various points throughout the long reign of the RPGA I was rewarded with a branded battlemat, the Fist of Emirikol, and two free adventures.
The battlemat saw a lot of use during D&D3, but now I use laminated poster-size inch-quad-rule paper; it's easier to see and erase. There are so many superior options to the traditional vinyl battlemat these days that I would be kind of disappointed to receive one, not in the least because D&D5 doesn't need one (praise be).
The Fist is awesome, and I still find a use for it from time to time -- for a while players in my public campaign could accumulate reward points towards rolling it and receiving a random advantage in play -- but as merchandise goes I would have preferred a nice branded dice bag or set of premium dice. A t-shirt, a car magnet. I still have my brass lacquer pin from when I first started paying RPGA dues back in the early 90s, and I still wear it on my dice bag. I like swag that lets me show off that I'm a "qualified" dungeon master.
(That's a dungeon master who meets qualifications, not a dungeon master only when certain qualifications are met. Isn't English wonderful?)
I never ran either of the two free adventures I earned (The Village of Hommlet and the direct conversion of the Tomb of Horrors) because by that time I was already through with D&D4, but all things considered I would have preferred some kind of balance at dndclassics.com had it existed at the time, so I could purchase my own choices of adventure. This is only more true now that D&D5 lends itself so well to early-edition conversions.
So, in short, I like branded flash and free money.
But oddly, what I'd most like to see come back is the ranking system from the old days. I was too young when it was discontinued and never had the opportunity to become a paragon judge. I realize this is probably counter to the mission of the Adventurers League and the simplification of organized play, but man, I eat this rank stuff up. That would really keep me dungeon mastering.