i'm rereading homestuck and got to the part where Rose enters the Medium; what do you think her quest was? how would she have gone about "playing the rain" and what would that have done for her as the Seer of Light?

It’s hard to say. I always imagined it was somewhat like, literally playing the strings of rain as if they were a violin, letting her Light Majyyks flow through the Land and bring a spark of life back into it, but it was left vague enough that it’s difficult to say. A big point of Rose’s Character Arc is the refusal to go through such a quest, and then the doubt after the fact about her place in the Universe and as a Seer of Light and what she should’ve done in retrospect.