The way I see it there is no gap, based on the way the tech tree works.
The archers role is as a support unit to counter the mele units, especially on the open ground.
In the time of swordsmen this was done by archers, in the time of rifles this was done by gattling guns.
As the longswordsmen does not upgrade to muskets, then I see no reason for crossbows to upgrade to something either.
That being said I would love it if they implimented a old fashioned mortar, with a range of 2 and indirect fire, that could fire without setting up.