The downfall of all time traveling series is that once you get past the major points of history (Titanic, Lincoln, Pearl Harbor, etc.) theres no where to go and the plots begin to become more outlandish.
The only series to generally avoid this trap is Quantum Leap.
So, I hope the producers sat down and developed at least 100 story ideas, before the beginning of production, lest Timeless devolve into another Time Tunnel.
There are hundreds or thousands of "major points of history." There's no excuse for running out of good stories when you have all of history as your setting (just ask the Doctor). I think what you mean is, "once you get past the cliched moments of history." I.e., "We've tried to kill Hitler and we've tried to stop JFK from being killed -- now there are no stories left!"
I found the first episode of Timeless quite disappointing -- the action was dumb, the adventure had no defined stakes, and the three lead characters were unrealistic and unappealing -- but if the show fails, it won't be because history's cupboard is bare.
The only series to generally avoid this trap is Quantum Leap.
Depends on how clever the writer is.
Poul Anderson's TIME PATROL stories sometimes picked completely obscure periods of history because it gave him the freedom to come up with interesting plotlines without being tied to historical details.
In one story (set in midieval Italy), every attempt to "change history back" simply created a NEW version of history, precisely because history books were not clear on what was going on.
In another story, the debate is "Is this a crucial turning point of history or not?"
In a third, two teams of agents can't tell: which group's agenda will have the effect of changing history, and which will have the effect of restoring it?
Time Travel stories do not have to focus on "major points of history".