IMO, the differences between classes won't be many, and maybe the mobility will be affected by a "weight value", which is the sum of the weights of your weapons (if you're carrying more than one) and (even if this is less probable) armor. With this system let's assume the Titan has a SLIGHTLY lower mobility, at the same time he would be able to carry heavier equip without getting affected/without getting affected as much as the other classes. This system probably would work nicely and assuming that each class would perform better with a kind of a weapon (damage bonus on a certain weapon depending on the class?) would make more sense: Titan would be a deadly tank with a machine gun, Warlock has magic (or more than the other classes?) to compensate his lack of resistance/inability to carry heavy weaponry, Hunter would excel with sniper rifles and long range weapons. That's my opinion but in my head it makes sense