Wouldn't the game be worse if the terrain was identical? As in, the desire to control slightly better territory is surely a good driver of conflict and therefore engagement... The edges have both disadvantages and advantages - less people in range so you're perhaps safer but have less combat targets, less people to contest power banks but less power banks in range. If we put portals of some kind at the edge of the map, it would mean that the entire map was as connected to other parts of the map, so the terrain was less diverse.