Snape and Harry encounter dementors and are both nearly Kissed. The procedure is interrupted at the last minute (by Snape or Harry or a third party) and somehow both souls end up in Snape's body. Harry, soulless, is comatose (or otherwise unresponsive) and mourned by all, while Severus increasingly finds himself with strange dreams, unexpected attitudes, and odd reactions to situations and people… How will he figure out what's going on? Can the two souls find a way to communicate, and return Harry's soul to his body?
Of course, this could work just as well with Snape's soul ending up in Harry's body. Feel free.
The Horcrux could come along too for the ride or have been eaten by the dementors, your choice. Could happen any time the dementors appear in canon, or any other time point you can make it fit.
Bonus if Harry and/or Snape find a way to destroy dementors permanently (because I never liked the idea that the incarnation of despair was eternal).