Adam wants his own career and rightly so. Brian/Roger clearly aren't done even with Paul leaving. There is OBVIOUSLY an appetite for Queen+ Adam but perhaps not so much in the "Adam joins Queen" vein so really there's only one thing they could do to satisfy most folks.

Queen+ Adam Lambert live in concert, one night only. Rehearse for several weeks, carefully pick the right songs, put the concert on in America (where, let's face it, for now, Adam is only big in the US,) film it, release it as a CD/DVD and voila!