I think it'd be interesting to have maps where some tiles start with neutral armies. I imagine this wouldn't be too difficult to add in, considering that when players leave, it essentially does the same thing. This would allow you to have a map that changes dynamically as the game goes on. A strong defensive position at the game start may melt away once people start getting 8's. This would be particularly dramatic in random reinforcement games, as you could have neutral armies of varying strengths that can, on average, only be defeated after a certain stage of the game. It gives people defended spawn points, but could avoid scenarios where a draw is forced by having too few entrances.
Adding them to the map editor would be pretty cool. I can already see people making bomberman... And terrible maps with 8 spawn points, and neutral 8's everywhere else. But I'm sure there'd be plenty of cool stuff...