Make it equal to walking speed and I think it'd be fine. Except, of course, if you're looking to use "Get across this gap" as a challenge.
It'd have to take up more of a class's design space than the shadow monk's feature, since it could be used much more often (typically, it's possible that the shadow monk could have unlimited use if the whole campaing took place at night, say).
I'm not sure it could fit within a race like flight, though, because it does have some additional benefits, like helping to avoid opportunity attacks and getting out of grapples and other tight spaces.
But you never know with this sort of thing until you playtest it, so give it a whirl