I think Tychus is going to return in HotS infested. There was too much focus on him in the teasers early on, they were selling a statue of the dude too, and during WoL, he was a very under played character despite the focus that was given to him. He seemed about as important and Matt Horner or any of those other new characters.
Particularly the comment he makes, "Oh, hell, Jimmy. I don't like the (sights/looks?) of that." (referring to infested terrans).
The idea that he was shot/injured/died on Char puts him in a giant 'probably' category.
Personally, I think this would reinforce a very negative relationship between Kerrigan and Raynor and make him despite his choice of letting Kerrigan live.
Giving the dude some infested Zerg strengths could really up the ante of this above-average marine. But then again, a lot of like-bodied Terrans end up as suicidal freaks or zombie-esque creatures. He could end up being just another Easter-egg 'hero' too.
I hope he gets some deep infestation like Kerrigan got and have him come out beefed out with some Zergling claws and some gun-mounted tentacles.
Alternatively, I want Jim Raynor infested. Nothing sucks more than 'knowing' which heroes will never die due to popularity or being a 'main' character. I think Blizzard hinted at some known heroes biting the bullet with a 'you never know, wink wink' so it would be nice to really gut a favorite or two.
Zeratul also seems like a 'invincible because of the story' type guy. I hope I get to kill him ^_^
And you always know these heroes never go down easy making it all the more satisfying to dig into them to savor their destruction!