Have to agree, Lovecraft should be a Great Writer (and hearing Sean Bean read about Cthulhu would be AMAZING).
Also, John Steinbeck.
And of course, (this can be taken however seriously you wish) Dr. Seuss.
When they announced Australia to be added, I immediately thought "Steve Irwin - Great Scientist". He's a naturalist, close enough.
And though he's not a military man, Ned Kelly passed through my head for Great General.
Oliver Cromwell would make another Great General, if he's not already in there.
I also think a Great Merchant could be the late Satoru Iwata, since, let's face it, video games (and Nintendo especially) make a lot of money. A game developer Great Merchant might even provide a new, unique amenity in "Electronics" or something. (NOTE: I would've suggested Shigeru Miyamoto, but Civ wisely plays it safe by only using deceased figures (I think). That being said, Mr. Iwata only passed away in 2015, so it might be a bit too soon to consider this idea.)
WAIT, I think I just came up with the
perfect, and fortunately, still living, substitution: Sid Meier. Do you think he'd agree?