Medieval archers didn't look down the arrows, btw. They didn't use the modern stance; they used totally different methods to draw and loose. Warband's actually pretty inaccurate that way, but so are all the movies. I presume players would hate it if they used the real draw / loose style... because 99% of them have never seen how it was actually done, so it looks "weird".
And while some crossbows had a lattice for aiming and a rough guide for ballistics, most didn't. More importantly, the aiming procedure for anything past 30 meters or so with either weapon would make trying to aim it like a gun a fairly unpleasant experience. I'd really hate having the crossbow model block most of my sight picture as I aimed up enough to correct for gravity for a distance shot, for example. There's huge difference between doing it IRL and doing it in a game, in terms of how it feels, because games don't deal with parallax anything like correctly.
In short, doing either thing in a way that would make FPS "iron sights" enthusiasts happiest would both be unrealistic and ahistorical, and, in the case of crossbows, rather unhelpful in terms of accuracy- which isn't realistic; medieval crossbowmen could achieve pretty decent accuracy IRL if they were well trained.
Aaaaanways... that two cents aside, you could do it with either by making relatively small adjustments to the current animations.