Sorry about the double post but:Physics class was a long time ago, but I'm rather certain your motion potential is still determined by applied force, otherwise anything would be able to achieve FTL. But, getting back to the original intention of this post, there's plenty of potential left to explore without taking away from the fun factor.
The limit here is due to the force required to increase velocity at near c speeds. as you approach the limit the mass of the vessel increases, due to the kinetic energy being so large as to introduce a non-negligible mass, the closer to c, the larger the mass and thus the greater the force needed to accelerate. eventually a limit is set by the engine power (and fuel reserves). However, this limit is still around the 0.9c or above mark. At v=c the mass becomes infinite (and other entertaining time dilation and relativistic effects take hold too!)