The way I see it, yes, most people would want to run mechs. It's Battletech and mechs are the big draw. I don't see a problem with making it a class choice situation, same as aerospace pilot, armoured infantry, tanker, etc. Each of those professions require years of training to become truly good at (and the point of MMO's for the most part is to play the heroes, not the regular cannon fodder) so it is reasonable to me to just pick what you want to use as a class choice. Want to try something different? Hello alt character. Perhaps combining general infantry and vehicle use into one class, as unarmoured infantry don't stand up well in mech combat unless there are hordes of them, or they have vehicle support.
The potential for RP elements to work in is pretty large as well. Choice between which military to join, or to become a merc, each offering access to specific equipment and resources. Military units get equipment and upgrades based on rank and mission performance, whereas mercs have to buy, salvage or steal what they need.
The key will be to focus only on the popular point of the BT universe, ie. the combat. You want to be a dancer or a cargo hauler or a miner? Go play something else. This is a game about war. War with big mechs. Focus on that, and make it the best in can possibly be, and this game will be a hit. Lose that focus and try do too much, and it will suck balls.