I know it won't happen, but...
...it would be awesome if the post-credits sequence were a close up of a gem. It's one we've not seen before. It begins to glow. Camera pulls back, and we see that the gem is affixed to a desiccated body. Suddenly, its eyes flutter open, as if woken by the other six Infinity stones...
Rune!
Fade to black.
Rune!
Fade to black.