First a few questions/ problems:
how is it that the battles just happen to take place where the Chasmfiends planted themselves? Especially considering that the Desolations were usually attacking the humans, and the Chasmfiends could hardly get that close without the humans noticing and killing them. It is possible that the last desolation was an attack initiated by the humans, and usually the Chasmfiends walk to the fighting, but otherwise this doesn't seem to quite add up. Also, is it really that much of an upgrade? All that time vulnerable, when you could be killed, in exchange for stone skin and faster movement doesn't seem too amazing since the Chasmfiends are pretty BA already. Also, what about all the other Greatshells? Do they all become Thunderclasts, do only Chasmfiends pupate? The theory starts to look a little iffy to me, except for "what else could the Chasmfiends be changing into?" However, if the Thunderclasts were going to battle, shouldn't their little bros be marching with them? Like, anyone who's already turned into a thunderclast behind goes sure, but the Chasmfiends are pretty dangerous themselves. Leaving them behind seems like a waste, especially when they have like thousands of years to replenish in between desolations. Therefore, there is no reason we must have already seen the result of a chasmfiend pupation, as we did not see any chasmfiends in the prelude either.
Then a couple crazy theories:
Sorry, no answer for that, but I do have a couple theories on the Thunderclasts: perhaps the Parshmen/Parshendi dead bodies are actually what drops the seed for the Thunderclasts. This would be interesting as each desolation would leave dead bodies that would drop thunderclast stuff (idk what it would be, but I'm sure Brandon could come up with something) that would mature into a thunderclast over like a thousand years. Then when they come back for the next desolation, all the thunderclasts that were planted rise up which helps explain why the parshmen are so anal about their dead bodies (more time in the same place = more chance of becoming thunderclasts, and the reaction to Kaladin was just an offshoot of dead body protection) and would support why the Desolations always caught the humans by surprise, because the thunderclasts were rising out from the very ground beneath them.
Or, what if the thunderclasts fly into the battle? OK, not like they can fly, but Odium or surgebinding or something launches them to wherever the battle is. Then they hit the ground so hard they're buried into it and thus have to rip themselves free of it. This would also explain Thunder = come from the sky + loud noise when they hit the ground. Do you think the crater/ blast radius would ruin this theory?
And then the theory that I mean as an actually solid theory:
What if Parshmen/Parshendi are the equivalent of Humans, and Voidbringers are the equivalent of Knights Radiant.
Just as windrunners are an order of knights radiant, dustbringers are an order of Voidbringers.