As a Valley boy who lives 10 minutes away from USH, I can honestly say that they'd have to somehow buy up a lot of developed real estate to add IoA out our way. Although that doesn't mean they couldn't find space somewhere for it in Southern Cal. But still...
Even though we're a bit more spread out, we're a lot like Orlando to Tampa Bay--lots and lots of theme parks to choose between from San Diego to Valencia/Santa Clarita.
And since I've been living here, two have closed down--used to have a Busch Gardens in Van Nuys and a Marineland in Costa Mesa. Granted they both closed 15-25 years ago.
In my opinion, IoA would have to be built full-scale to compete with DL resort, Knotts, and Six Flags Magic Mountain--all of which are well-attended mainstays with comparable attractions.
I'd prefer a few more rides at USH, being that they are practically giving away all day passes to get people in the park. Mummy addition is cool and will spike attendance. But what about Jimmy Neutron, MIB, and Spiderman? It's not like they don't have space for one or even all three. All they have to do is what they've done in the past--get creative in their ride spaces--a whole area of the park is built over a parking garage. I believe the T-2 3-D resides in this area.