I think it would be cool if the bosses got stars after you beat them. Maybe once you have all the trophies up you could face them again for bonuses. Here me out.
Once you have all the trophies up for bosses you could begin farming for ability upgrades.
After beating the boss again 10x and giving 10 more trophies you get the 1 star version to spawn at a different location with higher health/ different color and maybe new attacks and animations. After defeating the 1 star version 10x you can fight the 2 star version and get bonuses for the abilities.
Normal: use less stamina when running
1 star: adds use less stamina when attacking/using tools
2 star: adds increased carry capacity by 100 and increase duration/ decrease cool down. Or make jumping take no stamina.
For The Elder:
Normal: cut trees more efficiently
1 star: adds mine more efficiently
2 star: adds increased duration/ decrease cooldown and makes wood and ores weigh half their weight for the duration of the ability.
I haven’t gotten the other boss trophies but I think it would be cool to upgrade the abilities somehow. And would add some more to do.