i will get to this improvement one day. looks pretty easy.
civ allows for 2 different era models. if you add 1 third era model you run into issues you must address.
you can do more era models if you over come that issue. the Lua method swapping it to a new improvement once the era is hit is easy to do that's gives you 2 more eras to work with each time, with out having to do much work.
a great example of this being used is the village to town or trading posts to towns or one of the several variants. the only difference is the lua incorporates a counter system. if you know very little about lua or want to be able to still possible use the counter system later you can just change the thresh hold to 1 and change the improvements used in it to your improvements. if you keep the counter system up the graphic change is delayed by 1 turn but that's not to bad it just changes the graphics the turn after you enter the era not the exact turn you enter it.
steamcommunity.com
what 2 or more eras do you want. this looks east enough i can just swap out the buildings for different era buildings real easy and swap out old boats for more modern ones.
i can do ancient and an industrial version.
ancient and modern.
i cant promise nothing but it looks very do-able as at least a 2 different era model, if you want it to be easy and not go through the hassle that come with the additional eras.