Right now (VCMI 0.73c), we can start a map with multiple Capitols if set so from the MapEditor. And moreover, if we conquer an enemy town with Capitol, we can keep that one as well. Both of these behaviors could be kept as a modded feature (if enabled from the VCMI options), but I believe the basic game should keep the “1 Capitol per player” rule.
LE: After testing in H3C & WoG I was surprised to discover that H3 actually allowed more than 1 Capitol per player if built from Map Editor. It’s not sure if it was an intended feature, but nevertheless, that means we definitely have to keep this possibility, at least as a selectable option.
However, I think there is room to enhance a bit the gameplay, w/o having to alter the “1 Capitol per player” rule:
87. (NEW) In H3 the Capitol of a conquered town just disappears (if we have one already). Perhaps it may be interesting to actually get a message after conquering the town:
If we already have a Capitol: “While conquering your enemy’s town, you destroyed their Capitol.” (or more customized - “While conquering [town name], you destroyed [player color]'s Capitol”)
If we don’t have a Capitol: “By conquering your enemy’s town, you also gained control of their Capitol” or “By conquering [town name], your kingdom now gained a Capitol”
88. (NEW) A step further from the above, would be the open possibility to switch Capitols when we already have one and conquer another (it would represent minimal modding, but as it respects the 1 Capitol rule, perhaps it’s acceptable in the basic game). So I am talking about a choice message window after the battle: “While defeating [town name], you discover it contained your enemy’s Capitol. Would you like to destroy their Capitol or move your kingdom’s Capitol in the conquered town?”
The added value of implementing this, would be creating the possibility of better strategic decisions, for example in case the conquered town is placed in a location where we can provide better defenses as compared to the original Capitol town.
89. (NEW) And related to the above, for the same strategic purposes, it may be interesting if we would be able to dismantle a Capitol, in order to build it in another one of our towns.
I understand that this last suggestion starts to sound like WoG, but as long as we’re talking only about the Capitol, perhaps it’s acceptable. Otherwise we can just keep it in mind for when we’ll start working on VCMI mods. 
