I think integrating Nintendo Letterbox/Swapnote and Swapdoodle into Juxtaposition/Miiverse could be quite interesting. Imagine being able to read small handwritten letters of others and maybe even becoming letter friends with some you like! 1/2
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2/2 It keeps the randomness of Streetpass-letters but removes some of the anonymity, so that you can actually reply to it! App-integration could also be fairly easy; just add Miiverse as a new letter friend slot. Minimal game editing is required and the results would be fun!
P.s.: Maybe a new NSFW-flag would be required for Juxtaposition, since some individuals lack poetry skills and instead express their feelings in a different kind of way...