Let's think of this thread in terms of IF they actually go with the OGL, which mechanics from the OGC pool might be something to consider for the new edition. This would include cool creatures that you found in adventures, maybe whole adventures if they were 100% OGC, perhaps rule variants that you adopted for your own game, Maybe a type of weapon, magic item, or spell that you came across in a third party sourcebook, or some other such bit of Open Game Content.
If WotC does see the wisdom of making 5E an OGL game, what OGC from the vast pool accumulated over the last decade from 3PP would you like to see considered for adding into the new edition?
I honestly doubt 5e will be OGL. If so, it will be a very restricted license. They won't want to repeat the mistake they made with Pathfinder by giving their game to the competition.
Templates, specifically for creatures (not PC's).
If WOTC has them in 5E, great. If not, I'm hoping it will be relatively simple to make my own conversions. There are just so many good, nasty templates out there that can spice up an otherwise standard encounter.