I doubt that they would like to waste time and resources on creating such a mechanic - a more likely solution and cheaper to implement is to create various variations of the garrison or create a garrison development tree, where at some point we choose the option between two or three variants (choosing this and not another variation of the building for build, an unselected variant remains locked for the settlement) - this could also be part of the DLC distribution method - if you only have one dlc for the lords pack - choose a garrison from between the two variants, if you have both lords packs you already have a wider choice of variants.

Finally, I would like to point out - in no Total War title we could choose a garrison ourselves, never before - they have not done it so far, they will not do it now.

PS. If this is such an easy to implement idea - why haven't modders done it yet?