So my idea is a play on something already prominent in the world of Nirn which is Dwemer technology. I noticed that the some centurions have crossbows attached to their arm and my idea is simple. You got a request from a Calcermo to assist him in his research about the Dwemer society. One thing he is interested in is their weaponry that the people of Tamriel today do not have and you are sent on a quest to retrieve pieces of their weapons and he wants to study them and attempt to reconstruct them. You travel into a Dwemer ruin that's bigger and more underground then any of the other ones you've explored in Skyrim already. The point is to go to the different rooms and collect books containing data about the crossbow and parts for a prototype that Calcermo can use. Once collected, he willl than build a low-grade model for you to test out and if it all works out, then crossbows should start to become more frequent in Skyrim, but not as much as bows of course. There would be as many types of crossbows as there are types of metals (ie: ebony crossbow, steel crossbow, etc.) and some unique ones.
I have more, but I gotta work on the idea first before I post it. I know it's not the best idea, but I felt that there wasn't much going on in the game when it came to ranged weapons so I felt that a crossbow wouldn't be too out of place in the Fourth Era of Tamriel, I mean it's been centuries and they still rely on the same weapon types as the Third Era.