The obvious answer to the "What event will replace E3?" question is Quakecon.

1)It already has a huge following.
2)It is in the US.
3)There have been E3 sized displays at the convention for years now.
4)There seems to have been some word from ID software that they are willing to let more companies throw up displays in the future: http://www.next-gen.biz/index.php?option=com_conte nt&task=view&id=3596&Itemid=2