Good luck with this one Sept. The NvV games were great
As far as I am aware actually making a map from scatch inc. the map picture will be a lot of work. While simply making a map file with set starts and perhaps some special sites and possibly boosts for the initial AI armies is far easier and quicker.
I am also not sure of the virtue of a symetrical map in these circumstances. You have a team and your opponents won't care if they are being cheated or boosted. Mixing in sea like you are is likely to help human players more than AIs (who may not be able to handle the switch between the two well) and gives a huge boost to amphibious player powers. Remember what happened in NvV3? Those powers with easy access to water could get a dozen or so extra provinces almost for free. Even one AI sea nation changes this. And the choke points are both probably in the wrong place (not where the AIs will meet the players) and even if they were will simply be a big boost for the players who will use them and artificially boost certain powers with good PD.
So if you do have a problem getting someone to design a map for you then just pick a map you like and ask someone to set the starting locations for you.
I could do this. I can also make the AIs allied, add some special sites (if desired) and give the AIs some starting boosts if needed/wanted. I think the AI Pretenders can also be designed (but I am not sure) so I can probably give decent scales or blesses to the AIs.
But I can't design the whole map.