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)
Reading "Tattoo" makes me think of Monk first and foremost, and I would love a monk-ish warlock. That, combined with the celestial patron (another one on my wishlist) would make an excellent Disciple style Warlock.
While I would like them as well, I doubt we'll be seeing more invocations. Pretty much all the other caster classes that have been previewed so far have simply had new ways to expend spell slots or curated spell lists, and I can't remember any new spells being released, which I think invocations would fall under.
Patrons seem like the more likely go-to option, but they're just tied to high tier magical threats, and that's a pretty wide crap shoot. I think genies are a decent contender, and I would not be surprised at all if a vampire themed patron came about, which would actually be a decent way for PCs to live out that fantasy without being overly disruptive. I hope they avoid dragon patrons, it's too low-hanging fruit for my tastes, and steps on the toes of the dragon sorcerer, who is also a charisma based caster.
What I'd really like to see though are new pact options, which could give more leeway in bending the class. I think a swarm-focused option could be cool, as a different front-line option to blade pact, and it wouldn't be too hard to thematically link to the patrons imo. I'd also like an option for dedicated summoners, it's weird to me that the class with the strongest thematic ties to dangerous, otherworldly magical entities isn't very good at calling them to their aid.
When 5e started, I really expected that the chainlock was going to be the big summoner, and I was pretty disappointed with what we got. Last week someone asked Mike Mearls on twitter about why so few spells like summoning or animate dead in 5e, and he said they were too powerful (http://www.sageadvice.eu/2017/02/03/why-fewer-spells-like-summon-monster-and-animate-dead-in-5e/). That makes me think it is unlikely that the warlock is getting a summoning boost.
Personally, I would love a couple more Invocations.