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I've kind of wanted to do a Grass/Fire/Water legendary trio to mirror the starter types, but I'm not sure what direction to take it in. I was thinking something like Birth (water?) Life (Grass) and Death (Fire?) but the Kalos region already has life and death legends. Any ideas for other themes a Grass/Fire/Water legend trio could capitalize on?
How major are you wanting these legends to be? Is it a minor trio like the legendary birds or the Swords of Justice, or is it more dialga-palkia-giratina big scary gods of some fundamental world/universal concepts? The fact that your original idea was birth/life/death makes it sound like the latter, but just in case you could probably spin most of these to work for minor legends:
Heaven/Earth/Underworld
Maiden/Mother/Crone (from a common theme of “triple goddesses” in mythology; several examples of this as well as some other triples can be found here)
Perseverance (grass)/Passion (fire)/Pliancy (flexibility/adaptability, water), or in general tying those types to other qualities you might associate with them
Different kinds of renewal/rebirth/destruction (drowning/washing away, burning/purifying and leaving things to crumble and be reclaimed by nature/regrowth)
I would do a general search for interesting concepts that come in threes, as even if they don’t seem to tie in well with those three types at first you might be able to come up with something.
Since you’re already kind of tying the trio in with the starters, is there any way you could borrow more from the starters while looking for a theme? Are your starters’ concepts meant to come as a set (e.g. chesnaught, delphox and greninja representing the fighter, mage and rogue RPG character archetypes)? If so, maybe you can expand on that with your legendaries.
Lastly, you could also just not worry about xerneas and yveltal and put your own spin on the life/death idea. Even in canon we have two unrelated legendaries associated with time: dialga to govern it and celebi to take advantage of it to protect the forest and those it cares about (and possibly more). If you can come up with an interesting take on life and death (and birth) that’s more specific to your region, for example going somewhere with the heaven/earth/underworld or maiden/mother/crone ideas above, or tying the life cycle in with important cultural beliefs there, you’ll probably be fine.
Any other suggestions, followers?