I can’t give a definitive answer yet, but I’m swayed towards either the adipose, catkind, or the matchmaker (from the Swipe Right audio).
I’m honestly not a science fiction writer (I lean towards other genres), but if push comes to shove I might take a New Who story and do an AU where that Doctor and companion are switched with Six and Peri.
The Twelve Doctors of Christmas anthology has a story with the Sixth Doctor and the Catkind. Maybe you could do a pure historical with a historical foe that the Doctor has faced before?
I originally wasn’t going to put Catkind because of that exact story (and I’ve already written a Six and Adipose fic), but I honestly don’t know what other monster to have him face off against xD. I’m not into vicious, violent monsters and themes.
That’s a good idea about a historical “foe”. I looked up Carl Sagan and, while he is in the DW universe, it wasn’t a Doctor he met. I’ll keep looking.
Well, @The9thCyberLegion made this thread, so he’s in charge, but he’s said there isn’t a deadline, and you can take as much time as you like, the aim is to just have fun.