Nice (and refreshing?) concept for a topic this one, but I don't quite have an answer.
I suppose the easiest way to do it would be with Deadpool, as (via Deadpool 3) he can introduce any of the other MCU characters and still not interfere with the MCU timeline (fourth wall and all that). As far as the actual MCU goes though, I'd like for someone who's had less of a focus thus far, but at the same time, we've gotta remember that completely new characters may not draw in an audience.
If they can work around the age element (or just have her older) then Storm - eventually becoming a love interest with T'Challa could work. Cyclops and/or Nightcrawler maybe? I'd happily leave Mystique out of this one though, we really don't need to see her again anytime soon, and dare I say it, but the same can be said for Logan.
The likes of Colossus (because of Deadpool) and Quicksilver (Age of Ultron) could get a little complicated too, depending on whether there's a multiverse element in play for the characters being introduced.