Ultron, see above
Weapon X, in the current state the Mutants find themselves in Weapon X would be a force to be reckoned with.
Hellfire club, just because Jean melted everyones face doesn't mean they can't reform (no kids please)
More Reed Richards
Since we are ok with bringing back dead characters now (Psylocke) my #1 choice would be Magneto. He was/is the most epic villain the UU has ever seen, plus he appeared in Spencers run on Ult X-Men.
1) Find a bad guy
2) Be Wolverine
Bring Back Ultimate Wolverine...again!
These are some great ideas. I'm a fan of Radioactive Man, and I like the idea of some cold war connections there. I was NOT familiar with Phalanx, but I'll definitely look up some issues...
Obadiah is an interesting thought. I think he could become something truly effective in a longer series.
(I of course am just musing with fellow fans at this point :) ). But it's great to brainstorm villains...
I also miss Reed and The Children of Tomorrow, they were bad ass.
And I think that they should give you an Ultimate Comics Iron Man ongoing, so you could make a one-shot about the new Jarvis and how awesome/frustrating his job is.
Can't wait to buy the trade and show it to my friends, it's a really awesome piece of work.
Nathan tweeted that's he's outlining a sequel to something! Keep ya fingas crossed people!
I'm not Captain America, but I wish I was.