I would guess if they're going with DLC plus tweaks we'll get Adel and Oswald because they did some work on them and the assets are there.
I guess a new disc update with Adel and Oswald is a better idea then having them as DLC. That way, we can see more features other than just two more characters (Better netcode, English voice acting, voice acting in cutscenes, pre-fight dialogue, endings, and story mode (Silence kills the fun on all of them, and only the original Japanese voices really don't help me enjoy the game much, a little more vocal clarity and less Kansai accents will help me enjoy it more), more stages, etc.)
KOF XIII does deserve an updated version because of its popularity in both, Arcade and Consoles. A Vita version of the update is also welcome too.