It should be possible to jettison fuel out of fuel tanks somehow. Airplanes do this when they are too heavy to land. Rockets could also do it for similar reasons or if the fuel tanks are upsetting the center of gravity. In KSP this was more of a problem, if the ratio of fuel and oxidizer was wrong, but I digress.
The same future should also allow a tank to fill and dump water, making practical sea vessels and submarines possible to build. There could be three types of vents: passive (where the fuel leaks out by gravity or its own inertia), pumped (particularly important for submarines) and pressure-fed (a cheaper, but weaker form of pumped).
*feature, not future.