Well, it sort of seems like this season of The Flash is headed towards yet another timeline fuckery, and Legends is all about timeline fuckery (I believe, stopped watching after a couple episodes in Season 1), so it is entirely possible that they implement some sort of dimensional fuckery as well and bring Kara's world into the rest of the CW-verse. It would work well from a crossover standpoint too, because they could do another crossover of some sort at the end of the season, and possibly even the start of next season. I'm not sure how well it would work, but can you imagine a 4-night crossover to start next season? That would be pretty fucking cool if they did it properly, and might convince a few people to start watching more of the shows if they aren't already. The marketing and hype on that would be fucking insane, if you think about it. And once they are in the same universe, they could start tossing in random cameos/crossovers with just a character or two, like they did with Flash and Arrow at the beginning, so it isn't a big production each time, yet still makes a few people decide to watch the other shows. And plenty of times Arrow has fought people Barry could handle in a "flash" and they just ignore it, or at worst say something like Barry was busy with <Flash villain of the week>. Wouldn't be that hard to imply that Supergirl has her own shit to deal with most of the time too.