There isn't any canon evidence they didn't wind up together, either, so you can do it however you like. Sanosuke said he would eventually return, he and Megumi are at least friends, it's not impossible to imagine them eventually getting together. Of course, it's not impossible to imagine them getting together with completely different people, Sanosuke returning with a foreign bride, or finding a nice Japanese girl, and Megumi going after a rich man, or a doctor she meets, or someone she just happens to meet and fall in love with. Or, for that matter, neither might wind up with anyone on a permanent basis. Who knows?
Etna from the Disgaea series raising Hotaru. My thoughts for this...from what I understand, Professor Tomoe created an unusual space/time with his experiments, which had the result of attracting Master Pharaoh 90's attention from elsewhere in the galaxy. What if he managed to get in touch with a netherworld, instead, accidentally sucking in Etna, or possibly some other demons along with her...or Flonne to guilt-trip her into actually being helpful. If the accident still kills Hotaru, well, healers, presuming there's one along (anyone bring a base panel?) can bring people back from the dead in town...so, it might be possible. Alternatively, if she could get in touch with the Dark Assembly, they could reincarnate Hotaru, possibly as a demon...but she should still have the soul that was sent forward, so, she could probably still transform with the aid of her sailor crystal. Plus, imagine Flonne and Etna loose in the world...and without a Gatekeeper, they couldn't get back, though Laharl would probably eventually track them down.
...Ami being raised by Jenny from the Disgaea series would be interesting, too, or at least acting as a mentor. Though, Makoto would probably like her as well, since she's a kung fu expert, a genius, and an orphan, though one that got adopted...
