Fun concept: Nausicaa and Trigun are part of the same universe
This is an idea I thought of a long time ago. Basically, Nausicaa takes place on Earth, after humanity's attempt to rebuild our dying world into something new; free from the failures of our past. Trigun takes place in space after humanity's attempt to find a fresh, new world, where we can start fresh, free from the failures of our past. I imagine Trigun taking place earlier in the timeline, back when humanity still had hope. Maybe the failure of Project SEEDS was a historical event that added to the pessimism which led to the 7 days of fire.
From the two Trigun animes, it isn't very clear whether the plants are artificial lifeforms, aliens, or divine beings (although the reboot seems to lean in the direction of the latter). I never finished the manga, so I don't know if there's a more definitive interpretation there. However, if they are artificial lifeforms, that would fit perfectly with the Nausicaa lore. I'd imagine the ancient industrial civilization would have created some lifeform that can synthesize arbitrary resources. After realizing how hard it is to control the independent plants, they may have been split up into more manageable lifeforms, such as the Master of the Crypt, the God Warrior, and the Master of the Garden.
I don't really see this as that serious of a theory, since "dying world sci-fi" has seen many interpretations over the years, so there's bound to be some overlap.