It’ll either be great or terribly messy
Did SFxT play like both a Tekken and Street Fighter game? No, it didn’t. I think Tekken x Street Fighter will play very similar to Tekken Tag 2 but with Street Fighter characters who’ll have much, MUCH more moves then they’ll EVER have in any Street Fighter game and have the ability to side step. Projectiles? They might end up being unblockable but easy to see coming, like all other Tekken projectile attacks.
Projectiles are not really used to actually hit, they’re used to zone. I highly doubt Tekken will keep that feel.
Technically, they can make fireballs work if they make it fairly wide, so that a single sidestep can’t avoid it. At least in that way, fireballs can still be an essential part of the footsies game at sweep range. Not to mention giving fireball characters some decent tracking or wide attacks.
I remember Harada saying he didn’t want to make Tekken with SF characters nor SF with Tekken characters, but a new type all together. With that in mind, if we get Tekken with SF characters, I’ll be pissed.