Supposedly one of the developers tweeted that using an Unseen (legal issue mech) wasn't the actual cause for the delay, but they really haven't said anything past that.
Having infantry and aerospace would be awesome, would definitely make multiplayer incredibly amazing and deep. Just imaging that learning curve for all three types, is just awesome. Faster mechs, while more vulnerable would still be able to duck into cover (relatively) faster then the larger ones, aerospace having to watch out for the armaments on some of the bigger mechs (unless they're already engaged on the ground) and infantry having the bonus of size letting them sneak around and help out however they can.
Probably going to be more of a standard mech on mech type of game though, which still won't be bad.
On the developer's actual site, they've got a twitter feed that's basically giving out info on the game but using it as an in universe source of info.
http://www.piranha-games.com/MechWarrior.htmlI'd say the mech they're piecing together might be a Dragon or an Archer, but maybe it's a new mech altogether they're being allowed to make for the game. Not sure if they've said or not.