It is sad to hear the show is ending, but all good things and all...
As many, I've felt Melissa was a
great Supergirl. She brought tremendous energy and likability to the show - only the show itself (except for a season or two) didn't live up to the potential she brought to the character. The last few seasons especially have just felt more hokey than serious, despite the serious nature of the stories.
And taken on a whole with the loss of Kate Kane and the dynamic they were building with Kara, it just feels like the era is coming to a close. Arrow is gone. Legends was little more than a joke. The Canaries may be in the coal mine. Flash has been getting lost in its plots, Batwoman is restarting from square one, and Black Lightning still feels like it's in a completely different universe.
Maybe the best thing to do would be to reboot the entire CW-verse! (I joke, I kid. Or do I..?)
Rather than a crossover, the Stargirl series could get its own teen version of Supergirl. Not that they'd have to wait for the Supergirl series to end, but it might simplify the marketing and licensing to wait for that.
Kara and Courtney were the same age (not counting suspended animation) in Justice League Unlimited, and they had a friendship and rivalry that worked well. Have Melissa play Kara Zor-L's mother in flashbacks, and avoid the whole issue of the mature Power Girl costume by having Kara keep the classic Supergirl look during her early hero years.
There's a way around this:
Kara's infected by orange kryptonite and is turned into a teenager, where she's played by Izabela Vidovic.
Or have her come from the new Earth-38, where she is Supergirl defending Midvale.
I would love to see a teen-Supergirl show, or Powergirl! I floated an idea a year or so past where Supergirl gets de-aged back to a teen, which meant Alex could also then play mom to her! It could also have a far longer run starting with a younger actress. Also, young adult shows just seem to have more fun , IMO.
Maybe wait until the Covid mess finally dies down and start-up a brand new show with a young Kara. I would even prefer they reboot and start completely fresh, without any ties to this Supergirl show or the entire Arrowverse. I think it has far too much baggage by now as it is.