I thought about this a few times in the past few months. Two ways to have the team-up done.
1. Matrix rip-off. The S.P.D. we've seen thus far hasn't really happened. It's only an advanced cyber realm that five ordinary teenagers and two space cops (Cruger & Kat) were trapped in while the real life Gruumm stores up the mental energy that they've been exerting for the past year as well as the energy of all of Gruumm's victims in the past that unfortunately were never rescued before they became braindead. This would actually play off of the collective dream theories that Bridge is so fond of.
The Mystic Force team gets attacked by robot duplicates of the SPD Rangers, that use energy stolen from people trapped in Gruumm's machine. They end up getting saved by a gizmo developed by the real life Boom (or Piggy) who was Gruumm's assistant in the project but after a year of pretending to be Jack and the other's friends he sincerely became fond of them and can now no longer go along with the scheme. He gets the Mystic Force Rangers to rescue the others from the machine but they have to travel inside the virtual world first to recover them.
After defeating the cyber version of Gruumm once and for all, everyone is freed but now they have to deal with the issue of having an entire lifetime of fake memories and the reality that they were never real Power Rangers. This is resolved when the real Gruumm uses most of the energy he's gathered in from his machine to attack. The Mystic Force team is no match and the SPD team realizes that regardless of the fake memories and virtual battles their hearts are still those of heroes and they fight to save the Mystic Force even without any powers.
That is until Boom (or Piggy) and Kat, in an emergency use the last bit of remaining energy that Gruumm left behind to develop real SPD morphers. Jack and the others becomes SPD Rangers for real and help the Mystic Force destroy Gruumm at last. With their lives, theirs to live again they decide to establish a real Space Patrol Delta on Earth and for them their adventure has only just begin.
This move would resolve any glitches that S.P.D. has caused or will cause and sidesteps the problem of why we won't be seeing aliens immigrating to Earth for the next 20 years of PR.
The negative would be that S.P.D. fans may not be happy with the idea that S.P.D. doesn't happen but as far as the Rangers themselves are concerned all those battles and memories are a part of them and just as real as any adventure any other team of Rangers has gone through.
Besides it saves S.P.D. from being retconned out entirely which is what would eventually happen over the next decade anyway, simply because it's set in the future and no one is going to want to worry about adding elements that would lead to a season that's already aired several years prior. Especially not Bruce Kalish's successor, if and when he ever leaves the show.
The other idea for the team-up. *sigh* More time traveling. :shrug:
