Well, if they discover what it is that gives things 'mass' (and there are fairly sensible theories to explain it for every type of mass something can have at the moment (inertial, gravitational, relativistic etc.)) its possible that they may be able to create something completely 'massless' or which removes the mass from things inside it. If this is the case it would have a speed limit of lord knows what and would have no limit on acceleration.