I crossover I would very much like to see is maybe some sort of crossover between several different Sport anime?  I think this could be very cool and popular with sport anime fans, and there are many paths this sort of crossover could take.  

Like it could done as some sort of olympic event, or maybe even the characters from several different anime (Free!, Haikyuu!, Kuroko no Basket, etc.) could all meet up somewhere and they have to try the other sports.   Like maybe HQ cast could do basket ball (ROLLING THUNDER SLAM DUNK), and KNB cast could do volleyball.    XD

This is why it sucks to be swimming volleyball running basketball trash.  

Authors note:  What crossover would you guys like?