They did attempt an "Ultimate" universe of sorts. It was the "All-Star" line, which was not well received. I understand why it wasn't. In
All-Star Superman, Doomsday was... Jimmy Olsen.
All-Star Batman, featured a dark detective who exhibited a marked propensity toward calling an understandably confused and frightened little boy "retarded".
What-the-flamin'-crap?
If you don't consider the "All-Star" continuity as DC's attempt at an "Ultimate" universe, it's probably best that they never do. It would just lead to another dimension of realty and, with it, another excuse to cross-over every title into some damned multiversal, alternate-dimension-deleting crisis.
Seriously... It's like were stuck in a never ending loop that could be titled "Crisis of Infinite Crises".