Kotekuma I felt the same playing through Digimon Dawn/Dusk for the first time. There are some lines that can't evolve past even adult, like Palmon>MoriShellmon and Kotemon>PlatinumScumon. Sometimes I prefer the adult or perfect forms more than the ultimates anyways, so I don't mind if it ends there. I don't think I post any that don't end up as ultimate much here though.
I feel like the Monzaemon, WaruMonzaemon, Pandamon, ExTyrannomon and TonosamaGekomon don't need to evolve further because there's nothing that fits the theme of stuffed animal or feudal japanese Frog for Gekomon. I'd love to see evolutions in the vein of RustTyranomon and Titamon for them in the future though. I feel like the The Journey to the West Digimon would be a good example too, and a lot of the 12 Devas. I don't care that ExV-mon doesn't have a perfect that doesn't use Stingmon because V-mon also has Vdramon>AeroVdramon>UlForceVeedramon. Petermon not evolving would be fitting. And of course there's Culumon. Probably a lot of the recolours too. Add Drimogemon and Tortomon and Shellmon in here somewhere too but I feel like people have thought of good fan evolutions for these that I'm forgetting or something. When people use I wouldn't mind if they ended at that level. Kunemon>Flymon>HoneyBeemon is a random example, Digimon DS games had stuff like Terriermon>Galgomon>Prariemon too.
edit: new lines
Did anyone else notice Meramon and Agnimon have extremely similar poses? This has to be intentional considering how Agnimon has the fire element.
Here's a really obvious line for Meramon. The top is holy and the bottom is evil. I never noticed Asuramon's head is uncannily similar to the flame on Meramon's head. Asuramon and DeathMeramon are humanoid and Lampmon and Boltmon have green skin.
Does this line look like the most Adventure anime line for Lampmon?
Agnimon's line to fill the gap between Ignitemon and Bagramon. Bagramon's horns makes him look like he's supposed to have Greymon's horns.
A random one for Gankoomon based on Agnimon's card art.
Some random Jogress Digimon.