One of the weirder fanfic crossovers I have seen is Dylan Hunt is Hercules. Yes I know a Hercules/Andromeda crossover would be I don't know easy to fuck up? But still it was a fun bit of fiction.
Truth be told it's from what I have heard over the years. I wish I could remember where I first heard it. Apparently, Majel managed to put it together, with help, using some of Gene's older scripts and notes.