Im all for Mechanized infantry. Somethings I like from other games
Half track with 30/50 cal (mobile spawn point, Ammo, Meds)
Jeep with 30/50 cal (radio jeep. some sort of call in)
towable AA guns
There was a movie, The Bridge at Remagen that I loved the light mechanized company that stormed in as fast as they could.
I don’t think current maps can accommodate them easily but something to think about.
Are you sure? Because I don’t see bikes in Tunisia very often.
The premiums ones are awesome because you obtain engineer with mp40/thompson, very good squad to grind first few levels before you get engineer squad. But you cannot buy it anymore so it’s irrelevant. 
I don’t even remember last time I have seen someone using free bikes tho.
Halftracks/tracks would be good if they could contain other troop classes. Otherwise tanks could do everything better.
No I think not. They are easy to detect and you may lose the tank slot. So they could be really good balanced.