So that a realm can increase its tax base by signing peace with everyone and micromanaging its regions? No thanks - if a realm wants more cities, it should have to grab them from its neighbour. I like that the map is fixed - it's the politics that is not fixed, and that's where the fun is.
Furthermore, the current maps have a ton of faults that it'd be hard to make worse. The cities are often clustered, which means that after an unstable colonization phase, they are usually always held by the same powers, save for short transition periods during secessions. Luria Nova is a good example, it's a ton of super rich cities all clustered together, with vast plains, deserts, mountains, and sees to separate from everyone else. This kind of geography is overpowering, and you see this pattern repeated everywhere.
Dwi: Luria Nova, cluster of cities with great geographic barriers around, can take on the whole continent. If you think of Fissoa, though, it's only got one tiny cluster, basically no capital choice. Taking another city is impossible, because they are way too far. Politics don't matter much, they'll never become a superpower.
FEI: Arcaea, cluster of cities, took over the continent. If you think of Coralynth, the sole city is far off in a remote island. They have no alternative capitals. The other cities are so far away, that they can never aspire to become a superpower, even if they were filled with nobles and super active.
Colonies: much more balanced geography, cities are generally equidistant, realms are generally the same size.
BT: Rines, Grehk, and Athol Margos have, for almost all of BT, been in the hands of the same realm, Riombara. Invasions aside, of course. The map changes !@#$ed things even further, though. BT had generally rather good geography, but by taking Fengen and Enweilieos out (and completely depopulating Ete), it made it completely impossible to install a viable neighbor. With the destruction of its neighbors, and the screwing of the geography, the full South-East cluster could be complete, with the addition of Irombro, Jidington, and Eylmon. This creates an unstoppable behemoth, Rio's economic indicators are off the chart. But Grehk is the only viable capital. They are surrounded by vast barriers as well, and so the limits between them and their neighbors are somewhat fixed and independant of politics, much like Luria Nova.
AT: The fact that AT and BT use the same base map is really a great determination of how deterministic geography can be. Current AT and pre-invasion BT share almost the same realm borders. Except with fewer realms.
FEI: Also follows the major trends, most realm borders follow geographic barriers.
As it is, what realms are powerful and what realms aren't isn't firstly a factor of politics, it's a factor of geography. The way the continents were made, some spots are MADE to host superpowers, and others are INCAPABLE of holding them.