First, I'd have to agree with a previous post, and say that a magical thing would problably have a long, drawn-out, formalistic name instead of an acronym. The acronym could be a "shortening" of its formal name, though (whether it's the first 3 letters of a lengthy name, the first letter from it's 3 names, etc.). With that being said...
I think other background NPCs could be drawn from other sources (whether contemporary to the time, or "earlier" versions of more recent characters). Maybe the a few of the cast of St. Elsewhere or Hill Street Blues could pop up in the background somehow, or maybe a younger Det. Munch (ala Homicide/L&O: SVU) could be encountered (perhaps investigating a link to Baltimore somehow...).
I also think that this setting (with the heroes losing Secret Wars) is going to make things a bit rougher for the heroes still there. The villains would get a dose of wish fulfillment (IF they came back & the heroes didn't), not to mention how the Beyonder may act if he visits Earth ala Secret Wars II (and decides to be villainous himself).
The heroes not returning could kick off some events earlier than they should've occured. The Thunderbolts could appear in response to the heroes' absence. Tony (since Rhodey went in his place as Iron Man) could assemble a new group of Avengers; John Walker may be the new Capt. America for good; Ben Reilly may show up and take Peter's place. Of course, the PCs may be able to step up & take the place of the heroes who never came back, but it could be quite a tall order.