The road map says "more to come!", but I feel the same as you. I keep an eye on the plort market wall, though. There are five free spaces. The dev team makes waste of nothing, so I think that is an indication of their roadmap. If the wall changes and empty plort spaces disappear, I think that would be a conclusive sign we are near the end.
One spot may end up being the quicksilver slime. We may end up with a "Magnetistone Corral", and be able to farm quicksilver plorts on our home ranch. Or, Viktor may have a "Virtual Corral" that slowly produces plorts of any kind.
To Viktor himself, it sounds like the Glitch Slime and Glitch Plort are virtual items only, so they may end up being a second plort that we (currently) cannot farm. That is just a guess based on what has been said, so I could be way off.
Personally, I want a BoB update sooo bad, but I'm not sure there is official interest. There was a recent thread:
viewtopic.php?f=3&t=9017It has been a while, but I think it has been said that Thora and Hobson will never have their own side quest, and you will never meet them. This is a "never meet your heroes" philosophy, and to me it actually makes sense. Hobson has found his "happy ever after", and there is no reason for him to return. If we ever do meet them, I think it will be the closing act of Slime Rancher (barring a sequel). Just as a narrative construction, I think opening Slime Rancher with a letter from Hobson welcoming you to your new ranch, and the final act of the game being Hobson welcoming you to his ranch would work well. (Beatrix welcoming a new rancher would also be perfect closure.)
If the ranch exchange ever opens to players, then the last five spots may all be for platform-specific plorts for players to trade. Since the different platforms all have unique DLC, one possibility could be to let each platform have a unique island with a unique slime, and let players trade plorts. That could fill up the plort wall quickly.
All we can do is wait. There is already a placeholder for the Glitch Slime on the Wiki, but little other information.