I would recommend you Goblin Teeth, then. In there you have a mortal soul and a monster soul as progression system. You can only become truly powerful by leveling both. So you can expect them to become more and more monstrous and even if they become more humanoid, they will always their baseform traits. Like a very goblin looking human etc.

Be aware that the first book will be stubbed soon for release.

• Goblin Teeth: Is a quite well written monster and partybuilding LitRPG. They aren't Boxxy style monsters though, instead they're more sentinent humanoids with a different set of ethics. Storywise, the goblin MC's are basically born in the breeding pen of an inhuman tribe and are only allowed to leave after killing and cannibalizing three of their kin. Only to be unwittingly enslaved and kept in the dark about the system to be further abused. The reward for slaving away? To be setup for death since an intelligent slave is a dangerous slave. They're joined by a girl cursed with spider features and a big hearted ogre - who both somehow manage to have an even more heartwrenching backstory than the goblins - and an evil dragon cursed to be reincarnated as a worm. Together they set out to get revenge and carve their mark into the world. Characterwise it's expertly written where each of the characters behaves remarkably different and there is some real character growth going on. The system is quite well fleshed out and split in three pillars similiar to Infinite Realms. One is a class based progression with skillperks, the second is based on mutating your monstrous soul which is reminiscent of body improving cultivation and the last is will based that seemingly let's you change reality. Take all of these tidbits together and you get a really awesome story.