A new pact boon would certainly fit the big question nature of the weekly UA's, but it might be a little too big for a UA. If they did go that route, a tattoo/mark would be a way to do it, although I am not quite sure what powers that would entail.
I don't think they will make anything Dark Sun specific, but a slightly more generic Arcane Master patron warlock might work. Of course, if they did that, they might fold a wizard subclass into next week's UA like the rogue with the ranger.
I don't see that they would need to gage any opposition/support for anything like an elemental warlock or a shadow one, so I would be surprised if either showed up in a UA (my guess is that "easy wins" are going straight to the Big Book of Content)