My advice would be to pick 1 civ and make the next release about them. Focus on their traits, their units, leaderheads, diplomacy text, unique buildings, unique units, strength and weaknesses.
The advantage of this is that as a whole the work involved in a mod like this is overwhelming, you could go for weeks adding things here and there without any real game benefit for the player. But if you give them a complete civ players will be eager to jump in and try them out. You have something solid for the players to sink their teeth in and give feedback on.
That doesn't mean you cant throw in other ideas and stuff as you come across them (if they dont extend your release date), but by making 1 civ the target of your release you have a manageable goal and you can keep focus. You have incredibly talented people on this project, the challenge wont be coming up with ideas but focusing those ideas toward a common goal.
In the begining you may want to pick easier civs, certainly those that don't rely heavily on magic would be idle if the magic system isn't worked out yet. After getting a few under your belt it will be all the easier to start implementing more complex features or traits for later civs.
Again, just my advice, its certainly not the only way to run successfully but it may help you out.
Thx
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