I'm thinking of fics where Buffy/Spike meet their counterpart outside of the BtVS era. So typically Buffy time travels to meet William or where Spike meets Buffy pre-slayer. This meeting then alters how their relationship will be (pre-slayer meeting) or often changes the way it is (time travel).

The time travel ones are a personal favourite and two of my all time favourite fics involve Buffy meeting William and it then altering her relationship with Spike. The two I'm thinking of are...

Forward to Time Past by Unbridled _Brunette
Hearts Breaking Even by slaymesoftly
(both also on here)

In a similar vein, fics where Spike meets Buffy before she is a slayer and that affects their relationship dynamic such as...

In Omne Tempus by Holly
Sideways by goldenusagi
Future Childe by Ariel Dawn (slightly different anyway as it is a vamp!Buffy one but their initial meeting is still pre-slayer)

I've read quite a few of these because it is a premise that really appeals to me so I have my fingers crossed and eyes squeezed tight in anticipation that someone may be able to come up with ones I haven't found yet!!