Creating a scenario that is roughly historically accurate and provides a satisfying experience is a lot of work, more so the later it starts because there are more things to be added. Also, each scenario adds extra effort when adding a new civ and similar stuff. If anything, I would add a 1500 AD scenario first though.
Edit: if you have a GP name to suggest, please also post it in the corresponding thread so it doesn't get buried here.
Edit: if you have a GP name to suggest, please also post it in the corresponding thread so it doesn't get buried here.