Woah, that’s not an easy task you got there. So basically, you want the game to be a bit more lite Civilizations or Age of Empires. Interesting…
Welp, there’s pretty much nothing (or very little) in the game that will be of much help to you, even after the game is done I think. There are lots of features you’d need to add that would simulate that a new age has started. It’s an interesting concept, but nothing that I can help with personally I’m afraid (too busy with stuff).
It’s really ambitious to go at this kind of mod right off that bat. I’d suggest starting with something smaller, and then come back to this when you’ve got more experience with modding or when/if there are tools that could help you with this.
There are plenty of good advice on how to start modding in the following thread.
And then there are the mods that @sdee made as examples on how to mod (startermod_basic, and startermos_items). Taking a look at those will tell you what you need to do to create a really small mod that just adds a few items to the game. And of course, there are the streams where she made the mods: making a reference mod part 1, [part 2] (Twitch), making armor and buffs, and how to use the awesome devtools.
Not much for me to do but to point you in the right direction (which, hopefully, I’ve done).