I love Stargate SG1 so if they did put something like that ingame, I think I would be the happiest guy in the world. Imagine that, you could go around creating stargates on planets and enslaving the native races and creating statues in your creatures image, I think I would never leave my computer. What an awesome concept.
If not in the main game, maybe in a mod or expansion eh?
edit:
I highly doubt there will be a stargate of some sort in the game because that would make the point of the UFO useless, you can't really have both in Spore.
Sure you can have both. Actaully, having both would work very nicely. The UFO is like your swiss army knife so to say, you can have one creature of your species travel throughout the universe creating colonies and terraforming planets and the like. The stargate would be the thing that transports velichles and troops throughout the universe. So if you wanted to do a full scale land invasion, the stargate would be your tool to do it. Man, the more I think about this, the more I like this idea.