They could try another ultimates style universe but it would feel unnecessary. The movies fill that role already.
You could try doing what Bruce Timm did for Justice League Gods and Monsters. He kept the mantles and tossed everything else out (hyperbolically). In reality it was more of a What If/Elseworlds style story.
They could try rebooting the Malibu characters as the new Ultimate Universe.
They could always return to the MC2 universe. Kamala and friends could just be the generation in between the Silver Age Stalwarts and their kids. Miles is the Spiderman while Peter raises May etc.