I'd certainly be interested in possibly changing a starting mech or two. I'd be even more interested in knowing how hard it would be to add variants to the game, whether it would be possible to get them to show up in random missions, and whether it would be possible to make them purchasable. I would also very much like to be able to simulate XL engines with their double heat sink capacity, but again, have no idea how hard that would be or even if it is possible.
My idea would be, if possible, to have several mechs (ideally most if not all) with XL variants but make them such that they only appear 5-10% of the time, the rest being regular mech variants. The XL versions could also appear in shops, but be ten times more expensive than regular. The point would be to make collecting not just about finding the right mechs, but the right 'quality' of mechs too.