The only condition that is a must-have is that Spike says that particular line to Buffy and Buffy says back (however you want) “Is that a promise, Spike?”
Have fun with this one!
P.S. Bonus points if you get rid of Harmony and Riley in the process.
Special Note: This is based on a challenge from Sunnydalesis! Thank you for the wonderful inspiration, this has been one of my most fun forays into writing for this fandom yet and I owe it all to you. :)