The way 'Faith The Vampire Slayer' was envisioned was as Faith on a motorcycle travelling the world and trying to find her place in it.
Like 'Kung Fu', or 'Supernatural' meets Jack Reacher. 🙂
It's a nice idea and would've marked the show as different from 'Buffy' or 'Angel'.
Themes: Well, Redemption obviously, making peace with past actions/mistakes.
But not just that, I think the main themes would've been:
Home/Family (Faith is pretty much a loner, has never really had any family, yet she needs that in her life- and actually wants it. I imagine her seeing/interacting with other 'families'- like the people she saves, and searching for something like that herself.)
Relationships (This would link into the above, but more romantically. Faith has never been in a relationship- a grown up one- with the whole romance/closeness aspect. Learning to open up to somebody else should be a big theme for her, and it shouldn't take a five minute scene with Wood to make it happen. 🙂rolleyes🙂 It should be something much more meaningful.)
A Place In The World (Faith was there when the world was changed, but she really didn't have a part to play in the bigger picture, no big idea, no spells, no amulet. How does she fit in to this new world full of girls who are, basically, just like her?)
Searching For Angel (This would be a 'S2' theme, after 'Not Fade Away' on 'Angel', because if there's one person Faith would go to the ends the the earth for, it would be Angel. Finding out what happened to him, saving him from the macinations of W&H would have a nice 'quest-like' aspect we wouldn't have seen on the show before.
Lastly, what makes Faith special? We've had Buffy who broke the rules, we've had Angel, the vampire with a soul. But Faith?
She's always been the other slayer, the screw-up, the one who went off the rails. So, what makes her more than that now? What makes her unique and seperate from Buffy and the hundreds of other girls in the world. If the world is full of slayers then Faith will have to define herself as someone other than just 'the bad ass slayer'
Characters: Faith and two others to start with (like Angel season 1), with a few guest stars over the weeks (again similar to Angel S1) like Xander or Willow.
Since Spike is on Angel Season 5- hes out. (Plus, chemistry aside, the whole vampire/slayer dynamic had been done to death.)
Faith, Giles and Dawn probably.
Giles has been explained earlier by @Blaze.
And I always liked the way the writers were taking Dawn in S7, having her balanced between 'normal girl' and competent fighter. She would've fitted in with some of the above themes quite well (finding her place in the world while her sister was busy running slayer central, plus the romance/who are you aspects). Plus Michelle Trachtenberg really wanted to continue playing the character, and it would've been fun to see her grow up away from her sister.
(In the S8 comics Dawn is pretty much back to being a wasted character- suddenly wanting to be a 'normal girl' who goes to college, and who doesn't really want anything to do with supernatural stuff in S9)
Later on they could pick up maybe one other male character- probably a newbie (funny, Xander-like) who they could explain things too. Possible romantic interest, not sure about that.
Or maybe Conner or Gunn, coming over from Angel, could've been fun to see. Would've led to a bunch of Faith/Gunn or Dawn/Conner fic for sure. 🙂
Also of course Wood goes and gets brutally murdered in the first episode of FtVS. Either that or Faith tells him to go take a running jump, gotta undo that whole thing first, before we get to the show proper. 🙂