Alright, now that I've got a small novella going just in the comments to my post (*ahem*), I shall wrap things up by offering some plot ideas I collected from unclaimed Snarry-A-Thon prompts. If these fit into the holiday theme I mentioned above, great; if not, no worries.
In no particular order (go with whatever inspires you):
(1) Severus is hit by an unknown curse. When he thinks he's dying he confesses his love for Harry Potter (bonus points if he does so publicly). What happens when he ends up surviving and has to live with the consequences?
(2) Harry and Severus are sucked into a magical tome of "A Thousand and One Nights" and have to find their way out. (I have no particular attachment to this book, so you can select any other book or story that has a faraway, exotic or nothing-to-do-with-wizarding-England sense of adventure to it.)
(3) To taste sights. To feel smells. To hear touch. To see sounds. Upon defeating Voldemort, Harry's powers are thrown off balance. Now he must come to terms with his new reality. Harry is a Synesthete with all his senses explicitly connected. Using his new state of being, will his perceptions of everyone and everything change?
(4) Severus is becoming increasingly nervous by Harry’s non-so-subtle hints about bringing a third into their bedroom. What can he do to ensure he meets all of Harry’s needs while preventing himself from getting hurt - or worse, what he truly fears will happen - left behind? Here's an opportunity to make liberal use of jealous/possessive!Snape I'm so fond of. *grin*