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.
The only thing I didn't like about Bel was her inappropriate, almost futuristic outfit, perhaps. And also a common phenomenon for some "guest" characters and this is the reduction of MC.
Bel's character as a whole corresponds to the idea of the character and her motives to leave the kingdom, which lacked the freedom that she was able to get in the Neutral Territories. She essentially balances between extremes and knows what she wants. A narrowly focused fetish for the ass is obvious and this essentially cuts off those who have a fetish for other parts of the body. It is clear why it turned out this way: the fetishes of those who paid for the creation of the characters were realized.
I think that the main story lacks a giantess, who can have a similar position and have her own route of the Neutral Territories. It is not necessary that she be an alchemist and it is not even necessary that it be Bel, but it could be a replacement like the example you indicated, which I will quote below.
Rhys was so good she basically *was* added to the game, just changed up a bit (Makla).
But I like Makla more, not only in appearance, but also in general in her location, what happens in the route, additional characters. And her training has more meaning and is different from what was in the two routes with the dragonesses of the "guest" characters.
P.S.
If I were in MC's place in this situation, I would probably lose the lower half of my body. In the past, I was good at it and quite a lot, but long years of not doing exactly this... you can't just take it and do it right away.
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.
"Classic" dragoness, who guards her treasure from those who try to steal it from her, living in her cave outside the bustle of the mundane existence of other ambitious creatures. Of the two dragonesses "guest" characters, I personally liked Seinaru more in appearance, because of what could be seen in her route and I wanted to see even more. I am completely calm about the fact that there are people who make their choice in favor of Riis. It would be stupid to argue about preferences. Especially considering the fact that none of us decides for ourselves who or what will be liked and what will not. We only react to signals from the brain, we are informed by being presented with a fact, that's all.
P.S. And Seinaru also reminded me of Fafnir from the anime Kobayashi-san Chi no Maid Dragon.
Some of the appearance, character and obsession with treasure.
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).
Haramunia* is essentially a "hippie girl", a pacifist traveler who adheres to her own philosophy, she is unique and attractive precisely because she is.
I had to get used to her skin color for some time, which in my opinion does not really go with her pink hair and fur. But I do not consider this critically important so much that I do not like the character in appearance. Haramunia is essentially a good alternative to another travel lover — Sheina. And yes, from Haramunia's perspective, you could visit many places and meet different characters away from the main global events. And this is very intriguing in its own way. If I had to choose between Haramunia and Naya, I would choose the giant cat hippie girl, who also speaks funny, using the japanese "nya", which is literally "meow".