Additional tax on ports
Like many others I think the number of nations is less a problem than so many of the players being part of only one nation. Most players won't like it, actually, but taxes are a proven possibility to steer people's behaviour.
So, I propose an additional tax on ports.
Tax to be paid by every player in reals
Tax is progressive: the more ports a nation owns, the higher the tax per port.
Tax is only due on days the player logs in. (So, you don't get broke during vacation.)
Just an example with arbitrary numbers:
Nation owning 0 ports: no tax for players of that nation
Nation owning 1 - 5 ports: 1 real/port and day for each player of that nation
5 - 10 ports: 2 reals
11-20 ports: 4 reals
21-40 ports: 8 reals
41-70 ports: 16 reals
71-110 ports: 32 reals
111-160 ports 64 reals
These numbers would mean for Poland: tax oasis, China: 12 reals/day, GB: 80 reals/day, SE: 320 reals/day, and RU: 7.680 reals/day.
(RU owning thrice the ports of the second placed nation)
players joining the zerg for easy game will probably hate it
We have a sink of reals as the tax is lost to players. Should dampen inflation in reals considerably
Players get an incentive to join smaller nations.
Only ports that matter to a nation will be attacked and defended (is that a pro or a con?).
As a countermeasure you could reduce the fee for telportation.