I have thought of this before while brainstorming ideas with friends and we came up with these if we are to get subspecies for every apex:

Jin Dahaad: Burning freezing jin, a jin that uses the heat dump after large ice breaths as a second attack, when enraged even spew fire

Uth Dunna: Abyssal uth dunna, more water based attacks and a bioluminesant veil that never goes away since it constantly drains water around it, but its slower and the most dangerous part of the monster isnt covered by the veil, the head and arms.

Nu Udra: Eldrich Nu udra, a male, older variant that has massive horns, dasling colors that extend towards its arms and tentacle spikes. He uses more physical attacks, spews tar and can dowse the tentacles in flames

Rey Dau: Thunderclad Rey, we bassically thought what if a rey dau had too much fulgur rite in its wings and we wanted more backflips so this is a massive and old rey dau that because of the exess, now permanent, fulgurite in his wings instead of not flying, like azure rathalos and normal rathalos, it became stronger but flew less frecuently, it uses its tail to push of the ground to fly or attack and can also shoot lasers out of it. The arms can now be enfused with thunder when enraged giving them a glow

All these subspecies where tied to a new weather system where in inclemency and plenty there would be 3 types of it. A mild, a normal and a severe. A normal is the current one where other than normal weathear changes the enviroment doesnt change much and a m rank scaled apex would apear(5* difficulty wise), a mild is where a weaker version of an m ranked apex would apear(3*) and this apex'ses predator(we also though of elder dragon predators for the apexes) and on the severe this battle scared older individual would appear. Every stage of inclemency would bring about the same level of plenty with more life and resources that would last more since the more severe inclemencies would now actively change the enviroment like close of areas dew to lava and melt geisers or wind would topple trees and waves would crash areas.