I'm holding out for a hero the retcon ending, mainly in hopes that we'll get the glorious wordplay of calling the new game Saints Row 5.0.
Puns aside, it seems the most logical. As much as I want to have more awesome adventures with Jezzy and Zinjai and the old guard after the "new home" ending, it wouldn't be very compelling from a narrative standpoint. Throughout the Saints Row series, there's always been an overarching theme of conquest, of raising the stakes ever higher and coming out on top every time - the Boss has gone from gang-banger to gang leader to corporate mogul to U.S. President to all-powerful intergalactic ruler, and Gat picked up where Boss left off by sticking it to Satan and getting in good with God Himself. We've been through zombies, aliens, and demons. What's left to conquer? How can we continue the theme of escalation if there's nothing left to escalate into? The Saints supposedly had their fill of time travel at the end of SR4, and GooH touched on it as well by having historical homies like Vlad and Shakespeare, so that well has already been tapped. Not to mention that jumping from time period to time period would likely feel awfully disjointed in an open-world environment, on top of the oodles of dev time it'd require.
Another point to consider: what's on the line? Ultor and the Syndicate would have cleaned house if not for the Saints. Zinyak already took the situation to the limit by destroying the Earth, and Satan tried - and failed - to make things personal. I doubt a single planet full of aliens will be a worthy rival or provide any threat to a universe-spanning empire with Heaven on its side. It'd be a complete curb-stomp. To draw a parallel, as humorous as it would have been to see the Boss pop Loren between the eyes on that plane, or shrug off Zinyak's TK to deliver a TKO on the White House lawn, it wouldn't have made for a very interesting game...so as I see it, we as players would be better off leaving this Boss to live happily ever after blowing up aliens with their bros in their personal Valhalla, and letting a new Boss - or Chief, as the case may be - take the reins after starting over from scratch.
I think the police concept has a lot of potential, and it's possible Volition does too. If the new Saints are operating as the police force or a special gang unit, we'd already have material to bring back the gang warfare of pre-SR4 games. And since they're a government agency, it's possible they'd still have access to crazy technology allowing for antics like in the newer games. Maybe the federally-funded STAG (Stilwater Tactical Anti-Gang Unit? Eh? Eh?) is trying to move into town and put the Saints out of a job with advanced warfare, and you'd run raids on their bases in between gang action to steal their cool experimental toys like Grav-O-Boots or Power Gloves. Who knows? I have faith that the writers and the devteam know what they're doing.
End soapbox.