Herewith a quick signal-boost for an annual project from one of my favorite broad-spectrum fanfic writers:

jedibuttercup runs a "Wishlist" prompt fest every year at this time in which she accepts 24 prompts for stories (av. 500 - 3000 words) in the top tier of the broad range of fandoms with which she's conversant.  This includes the MCU [movies only; not the various TV shows], the Buffyverse, Leverage, Star Wars, and several other medium-to-large movie and TV series fandoms; for a fuller list, check out the link I'm about to give you.  In past years, this list has filled up in a day or so; for some reason, this year there are slots to spare, and it's always been a completely open exercise -- there are no pre-requisites to offering a prompt except reading and abiding by the overall rules, which are straightforward and not at all complex.  It should be noted here thatjedibuttercup is even more of a crossover hound than I am (and extremely good at pulling off unlikely crossover premises, although that's not -- or at least not entirely -- what drives the Wishlist fest). 

Here's the post announcing and collecting prompts for the current round, which as I type this has seven slots left.  (I am writing and linking this on the DW side; there are, however, corresponding LJ posts if that's where you live online.)