To me it's clearly a fast firing gun and a slash from either a sword or some kind of special weapon.
The gun is clearly between Jinx and Urgot, and since it was about exactly a year ago Urgot got a skin. It could very likely be him.
As for the Slash, there's multiple options I can see it being. First thought for me was Kayn's Auto. But the slash when in slow-mo clearly doesn't look like a scythe, so i don't think it could be Him. But just after gun fire stops You hear this "Wawo" sound which makes believe it's Kassadin jumping in with his Ult and does his slash. It's clearly a male as we hear from the sound So it's not Kayle. That's for sure. The only champion else than Kassadin is Pantheon. he hasn't got a skin in a long time and I could easily see a Project Pantheon.
Last but not least, there's a chance that this is a single champion rather than it being 2 of them. So I was thinking "What would match this?" And there was only one champion I could think of, Varus. "Now, now slow down. Varus doesn't use guns?". No he doesn't but, clearly legendary skins can remake weapons you know ;) Giving Varus a Gun and then that charge attack of his would perfectly match this trailer. Ofc it's a very little theory but it's at least one, right?
What's your thoughts on this. Is it Urgot? Pantheon? Varus? Or have the quiz only started? Who are these monsters riot are about to give us?