That would be a nice feature, more akin to Diablo II. However, EA seems intent on moving to closed systems regardless of genre. I can only imagine what Plants vs Zombies 2 will look like.
Hyperbole aside, it should be noted that Societies was developed by Tilted Mill, not Maxis. They also pinch hit for Sierra on Caesar IV, another sequel that deviated sharply from its predecessor. Contracting out franchises has mixed results (for example, I thought Dungeon Siege III was a comptent game esp. in light of Obsidian's history of buggy releases).