Do! And not only from the enjoyment of writing it aspect. I've taken two of my 300s and turned them into serials for Kraxon, so not only enjoyable, but profitable! I'm pretty sure Jo Z took one of her Challenge stories and made a novel out of it, and Phyrebrat has had Challenge stories published elsewhere and used ideas from them in longer pieces.my story easily wanted to be 1000 words. I do believe at some point I'll write the full version
Speaking of Kraxon, as I was, if anyone thinks to take a 300 worder and turn it into a 1000 word piece for the monthly story, acceptance for publication will depend on how the original story is treated. If the long version is simply the original writ large, with more description/dialogue/whatever, then it's unlikely to be accepted under the already-published rule. However, if the long story uses the original only as a starting point, and it grows into something very different, that's another matter. If anyone wants to try his/her hand at a submission then more details here Magazine Submissions 2021 and if more guidance about translating a particular 300 into the 1050 required would be helpful, just PM me and I'm happy to chat about it.