Just to touch on the subject of the KOF guest (since I'm late) personally Kensou would be a dream as he's my favorite KOF character but since that's a pipe dream we'll touch on the realistic choices.
Kyo - Face of KOF, K', ' Shun'Ei, Ash, all of these are/were popular characters but Iori aside wouldn't be chosen over K'. Plus out of all of them Kyo has the most versatile moveset that's changed throughout the series history that would most easily translate to DOA. No fireballs? Easy, use a variant of his KOF98 to 2002 moveset. The guy has moves for days. It would be unbelivable fun and I'd love to see him.
Iori - Not my first choice but he could make the cut as well. His popularity is up there with Kyos and while his moveset has seen less chances, the newer iterations of him from the last few KOFs (where he lacks his flame) could translate into a feasible moveset.
Terry - The most likely and obvious choice for a guy. Not much needs to be said here.
Rock - Dark horse, no one would see it coming and he has a style that would translate well into DOA. I could see it since his father is a guest in another fighter. Would be par for the course as the edgier son would be in a edgy game like DOA.
Then we've got characters like Kim, Ryo, Clark, Daimon, etc which would transition beautifully, but simply lack the star power and would never make the cut.
Let's be real though, this is waifu fighter. So it'll be a girl. So... most realistic choices here.
Kula - #1 choice. We've got the well endowed Mai, now we'll have the semi-loli Kula and the japanese would eat it up. Her Ice compliments Mai's fire and she's right in line with the type of characters being added to DOA right now. I'll be shocked if it's a girl and not this one.
Athena - #2 choice and my personal pick. She's my favorite female KOF character (yep, Athena and Kensou) and I'd love to see her transition to DOA. She's got plenty of costumes lined up which is perfect for DOA as she could have quite a few costume packs and her moveset is versatile enough to port over minus of course the Psycho ball.
Yuri - She's popular and like the others, has a bit of moe-ness which could contribute to her chances. That and she utilizes a bit of silly variant of Karate which opens up some possibilities of interactions with Hitomi.
Pretty much any other girl is far less likely to be added over the above 3. But fantastic choices either due to popularity or a great moveset/coolfactor/aesthetic potential are the likes of Vanessa, Blue Mary, Angel, Mui Mui, King, Mature, Vice and of course, Leona.
But... C'Mon, you know it's just going to be Terry or Kula. If it was Kyo or Athena that would be a surprise, but I'd be a happy camper. I'll buy Athena.