In earlier arcs of a GURPS Black Ops game (a sort of off-the-books anti-weirdness force, they are somewhere between Torchwood and UNIT) I have hinted that they would become the people responsible for the orderly keeping of the timestream in the era between the 20th century and an indefinite far future age. Previously, time-travel was by the plot device of an Edwardian luxury hotel that mysteriously allowed patrons access to other times and places according to how they signed the guest book, and encounters with the previous and successive "time guardians" were well-received by the players.
This was a fine arrangement in that the PCs had reasons to limit themselves to reasonable behavior without their being hard limits, and there were plenty of ways to indirectly handle excessive exploitation by having the employees' attitudes and quality of service reflect the party's treatment or mistreatment. Even the inevitable evil twins had to behave lest they get thrown out and banned from this unique venue, and there was always a possibility that extreme mayhem outside the hotel might follow you back to it or render the location unfashionable and closed for profitability reasons.
It has since developed that this hotel was essentially a rogue time-and-dimension-bending spacecraft (plenty of room inside, doors and windows look out on all sorts of places and times, minimal shaking and FX when kept in proper working order and operated with skill and care...from somewhere), subsequently destroyed and its essential mechanisms looted by The Empire of Space, whose soldiers all look alike under their domed helmets, then swiped back by the PCs, and controls gradually restored. The original power source has been replaced by several atomic furnaces, and preliminary operations have allowed travel to a few distant planets or eras, each so far inhabited by humans or human-like beings under harassment variously by blood-seeking silicon creatures and paranoid robot sentries.
I would like to give the PCs a bit more control over their comings and goings, but I'm drawing a blank on useful constraints for this sort of thing. I can steal some ideas from Stargate, like replacing "ZPM" with "atomic furnace", and right now they are using control systems based on the old Hotel setup, so they have to spend the night coming and going and hope that the computer's best guesses for dial settings take them somewhere interesting (no employees survived, so no one knows how the destinations were translated into manipulations of the unlabeled inputs on the central column), but that's all I have at the moment. There's not a big rush, I've taken off gaming for a while in favor of school projects, but when they're done, I want to be able to get directly to "the good part."
P.S. One thing I'm considering is to use the energy costs for existing travel spells and say that the energy is paid by the fatigue expended from long-distance hiking down miles of corridor and up legions of stairways; this worked well for an earlier adventure where the future guardians led the party back to their native time through time-tunnels, and again explained why the future guardians humped minimal gear, since encumbrance penalties didn't count toward the "spell". For those not familiar with GURPS, this could mean taking a week to walk to the door or stone circle that lets out 1000 years in the future, or couple of days if you find a more direct and strenuous route, like climbing the world-tree or crossing the rooftops of Dis. But with PCs doing it, it raises the question of how they know where to go, what happens if they change their mind part way along, etc.