Ok, so what kind of block would be a good fit for Pyrulea? It's a densely forested Dyson sphere with oversized plants (but apparently not animals?) What mechanical theme is there to explore that matches "plants here are, like, super big in comparison to animals?"

It's a similar problem to Regatha. The whole plane has high volcanic activity and reduces to essentially "shit's on fire, yo." How do you make an entire block out of the theme "fire?" Remember Magic has colors other than red.

Most planes have a single shtick, but not all shticks lend themselves to good MtG blocks. Kaladesh's "invention as art," Vryn's "two factions fight over control of mysterious technology," Valla/Kaldheim's "totally not Norse mythology" are all great concepts for MtG blocks. "Giant plants" is a much harder sell.

I'm not saying it's impossible. WotC can get pretty creative sometimes. I just think it's really, really hard. Pyrulea is the sort of plane that is a great gimmick for one or two cards in a product like Planechase, but doesn't lend itself easily to a whole block.