I would love to see this addition, with a caveat: trading should be limited to special event decor Pikmin and Seedlings only. A user in a separate-but-similar thread suggested this, as the point of the game is to push folks to explore and travel (i.e., keep us using the game longer). Allowing trading of everything would likely reduce time in game (bad for Niantic) and turn Pikmin Bloom into more of a shallow collection game (bad for players IMO).
I’m hit or miss on trading nectar and petals: there are certainly times where it’d be nice to share those with my partner for finishing challenges, but we’ve found ways to make it work (one of us using petals to force-bloom a big flower into the kind the other one needs).
But, I don’t see any of these happening as they would remove a revenue stream: if we can trade special event Seedlings, petals, or nectar, that lowers the amount of purchases in the Shop.