An actually surprising amount of the guest characters were pretty darn good- some of them were very blatantly self-insert mary sues and thus largely uninteresting (I think the alchemist's name was Bel, she was rough) but others could have easily been solid additions to the game.
Rhys was so good she basically *was* added to the game, just changed up a bit (Makla).
Seinaru is definitely inferior to Rhys, but she's got some cute aspects to her- she's definitely got some OC energy, but it's not suffocating.
Roxanne was fantastic, there's something amazing about a giantess with a height complex and the ability to grow/shrink others. Her toying with you (without being an evil bitch about it) puts her near the top of my list of guest characters.
Even though I see people complain about her verbal tic, I just shrug my shoulders- it's really the price of doing business in an even vaguely anime-esque game, but Harumunia's casual lewdness concerning you and others, offering you to ride in her panties, the demeaning-but-not-cruel way she calls you "tiny" or "toy" while not actually disregarding your personhood, all of these things have yet to be replicated in any other giantess in the main game. Add to that her whole shtick is being a traveler, she'd have been a great route for seeing all the continent had to offer and sampling a smorgasbord of different giantesses (that you'd likely end up shoved into).
Zanna was very good as well, for similar reasons to the above- kind, caring, but teasing lewd things- absolute blueballs that the good stuff was never reached. (List in no particular order of quality).