TheFlyingGreenMonkeyFrom the theories I know of, not many are infinite. -The ones put forward by serious physicists that is. :)
If the uni is truly infinite then every possible outcome has happened. That is the first theory. Another theory is for each possible action their is a universe for that. Then there is the whole pbrain theory saying that universes are actually sheets/membranes that sometimes rub/touch each other and when this happens a new uni is formed. There is one more main one I am forgeting.
For the SciFi fantasy ones though, anything's cool, but then most aliens in popular SciFi are human shaped, just with weird skin and a funny head -_-