I think that a mayor should be able to buy a small size change like this: normal portal | bigger portal this way people with odd sized towns/buildings can have non-awkward looking portals. the size change would bring some costs with it ofcourse so I was thinking of $5k per portal.
I believe Andrew mentioned a while back that maybe in the future we'd have custom-shaped portals, or other sizes for them available. I'm guessing it depends on what the plugin can do, time to test and make sure really custom-shaped ones work, then figuring out how to price it (assuming he adds more freedom than only one or two different kinds to choose from.) I've wanted this for a long time, since I've always preferred the corner-less portal some people use in MC already. ._. And that off-centerness in some towns makes me cringe a lot.