Looks like really nice changes! 
but now that the possibility for guarding choke points with cities is almost gone (I think its a bad idea to use cities like this anyway), I think it could be interesting if it was possible to build something like forts/defences.
Maybe a fort could be made up of something like 1-4 squares that you choose. The bigger, the more costly. This would also put more emphasis on tactics and navel landings to get around those nasty choke points
To avoid spam the forts could have an upkeep cost or a maximum limit.
I also like the idea that was presented earlier in the thread, about hero's asking to join your nation once in a while. How your empire attracts the heroes could be based on one or a number of things.
Work Based:
To avoid a slippery slope type of gameplay. One could argue that heroes would be attracted to where the work is at. For instance a specific type of hero would have a higher chance of being attracted to the empire who has the fewest heroes of his own type. Who needs competition anyway 
This type of attraction would also lessen the chance of an empire only getting heroes of the same type.
Research Based:
A hero is attracted to empires bases on the research it has chosen. For instance if the empire has researched a lot of magic. mages would have a higher chance of being attracted. This type would make research matter even more.
City based:
The hero is attracted to cities and not empires. For instance a warrior has a higher chance of being attracted to cities who has a lot of warrior type buildings like, training centres and buildings that produce weapons.
City attraction would also help prevent a slippery slope type of game play as a player being behind all in all, could still have a great city and thereby still attract great heroes.
By the way I'm quite new to the game, so if any of the things I have mentioned is already in the game somehow or have already been mentioned, I apologize in advance.
Keep up the good work.
