I don’t usually comb through Dashing don so I don’t know which projects never saw the light of day but I can think of a few follow-ups.
It’s not unfinished per se but my trilogy-minded self has a bit of s reaction to there being only two Deathless games from Max Gladstone. They were standalone but both really good and I was always hoping for one more game to round it out. @dfabulich if you ever chat with him, let him know! 
I’m also not entirely sure what was unfinished or abandoned. Other ones that come to mind:
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Lords of Aswick was excellent and I thought that I read somewhere was supposed to have a sequel.
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I always wanted a sequel to Saga of the North Wind. No idea if one was ever planned.
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There was a Starship Troopers-ish tale called Battle for Cerberus that I think was supposed to be a series. I’d love a multipart space opera.
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Choice of Uprising is another, but I believe that one is getting a sequel at some point.
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I don’t know if Psy High is getting another but I hope it does. I was really enjoying the world that Becca was building and both games were great.