Variation on an earlier idea:

After the most recent fun with thieves disguised as terrorists (which could be any of the movies! :D), John McClane gets a new partner. He keeps getting stuck with the undesirables because nobody else can handle his crazy, so it's not really a surprise that he gets stuck with the new guy...who just moved to New York from the UK. How much of the city will be destroyed in the name of saving lives before John McClane and Harry Potter get split up?

Willow, Jesse, and Xander used to do everything together. The only limit to their time together was their parents, and none of them had parents who even paid enough attention to notice if they came home every night. They had varying levels of interest in and skill at the activities they did, but they all did them- TV and comics and computers and whatever else they did. Jesse's death ended that. When they get together without Jesse, they feel his loss because always before he'd be there. So they latch on to Buffy as a convenient excuse (though they never talk about it) to not spend all their time together, at least alone.

Time goes on through canon. Willow may or may not continue their activities on her own. Xander keeps doing at least one of them: computers. I assume he has a reason for not displaying his skill. When he leaves Sunnydale, either after graduation or after Sunnydale becomes a sinkhole, he (eventually) sees/meets hacker friend(s) in person. Maybe one set of hackers for after graduation, and another after the First. [Hackers, Die Hard...do I know any others with hackers? Pretender, I suppose]

His handle is not something you'd expect, even if you know him. Possibly he's run into Willow (using a handle he doesn't know) online in the past. Possibly he's changed handles since Jesse died.

All ideas are free for the taking.

The guy from Accepted was actually Matt Farrell. Maybe something happens at/with S.H.I.T. and he asks for John's help? Or they run into somebody who recognizes Matt from S.H.I.T.

All ideas are free for the taking.

Harry Potter is John McClane

Harry Potter is beat up and badass after the War, so he glamours himself up and makes a life for himself as John McClane. Later, he ends up in an AR where the war is ongoing (HP dead, Lily and James alive) and settles in teaching at Hogwarts or something else where he's close to the action/the people he used to know and love. And then Matt Farrell ends up in the AR too.

All ideas are free for the taking.