AtS season 6, for the most part is in comic form (After the Fall--read it, folks), The only major difference is that they killed Wesley in Not Fade Away to give him closure, whereas he wasn't supposed to die until the finale of season 6. He's a ghost in After the Fall. Amy Acker (Fred) spoiled years before the comics were written that Gunn was supposed to become a vampire in season 6. Most of that series is what season 6 would have been. The road warrior thing was only in the sense of everyone being in Hell.
Similarly, the story that was supposed to be the Faith T.V. movie got pulverized and turned into No Future For You. And the Buffy Animated Series pilot was turned into After These Messages... We'll Be Right Back!
And actually BtVS wasn't supposed to end at season 7 either. It was Sarah Michelle Gellar who wanted out. And it's quite clear from season 8 that the story wasn't over and there was a whole mess of loose ends (believe it or not) towards the end of the series about why Buffy, in particular, was in such a rut (and had just learned that the Slayer origins were demonic). Also, Fray was written before BtVS ended, and if you've read that, you know that BtVS and AtS both need some massive follow-up to link them towards that dystopian future where demons/vampires were kicked out of the dimension (but leaked back in somehow).
And as for Angel's story post-After the Fall, well... Spoilersville. The Frayverse demon exile that I won't spoil here... The Shanshu question (an unfulfilled plotline which is now on its 10th birthday!)... Lots of things still left to be answered with him.
Spike is still pretty much a baby to his new heroic journey. That story has still barely begun. Ditto with Faith.
So there's one franchise that kind of all is one story (and lately it's been marrying itself again). 11 1/2 seasons plus two comic lines (and a whole lot of extraneous earlier comics which are both canon and non-canon).
And if you've seen season 4 of AtS, it never made fans of episodic television very happy (it happens mostly in 4 weeks--it's as unforgiving to just jumping in as Heroes can be). AtS, even more than BtVS, was known for barely taking a breather on its serial storytelling. And those shows like to reference things that happened years ago all the time.
I'm a serial television fan because I have the brain for it. I like intricate, self-referencing, lengthy storytelling.
And 5 years isn't enough. Comics that matter (which I follow for BtVS, AtS and Heroes) really help. Roswell, sadly, felt like a very short 3 seasons. I'm aware of canon books that followed, but they're apparently rare/expensive beyond belief.
I even like gleaning the could-have-happened bits from BtVS and AtS non-canon comics (I really have a soft spot for the year 1 comics, especially), deleted script scenes, non-canon books, etc...
I mean, I am on a Trekkie website here. You should relate! Star Wars was the first fandom that introduced me to the world of a fandom that follows a never-ending story.
I hope Heroes keeps going in some form. I think comics would work. That's already established.