(Please note that, unless changes are explicitly noted, Buffy canon stays much the same.)

Freshmen

Xander and Cordelia both are accepted in Fraternities

First year of college goes well (Xander can hardly believe it)

Spike gets neutered (as per canon), but starts getting to know Faith a bit

Xander goes back to Sunnydale over Christmas for one last attempt to mend fences, it doesn't go well.

Faith and Wesley move to LA, Spike comes with her and ends up staying at Angel's place

Oz breaks up with Willow (A oneshot tentatively entitled - Five times Oz wanted to break up with Willow, and the one time he actually did.)

Iron Man 2 happens, except differently. Tony and Pepper's ongoing healthy relationship changes a lot.

Xander and Cordelia spent the summer travelling to all 50 states (Tony's money helps)

Sophomores

Oz transfers to UCLA

Cordelia starts considering becoming a lawyer

Xander is doing very well at his double major (business and applied mechanics)

Faith finds out she's pregnant from one night stand

Xander spends the summer interning at SI (his identity is still a secret)

Juniors

Faith and Oz start dating, move in together

Both Cordelia and Xander become presidents of their chapters

Captain America wakes up

Avengers happens - not identical, but similar. Xander tries to surprise Tony with a visit on the day it all goes down and so meets Loki while he's waiting in the tower. Cue banter.

Faith has baby, Oz acts as baby's father

Tony and Pepper get engaged

Iron Man 3 happens very, very differently!

Xander spends the summer interning at SI

Seniors

Cordelia takes LSATs and aces them

Cordelia applies for, and accepted into, Columbia Law

Cordelia and Xander get engaged while in a life threatening situation helping Faith and Angel take out a big bad.

(Avengers: Age of Ultron doesn't happen)

Tony and Pepper get married - Xander uses this opportunity to finally announce his identity to the world.

Xander and Cordelia graduate from college and move to New York

And then...

Cordelia starts at Columbia Law

Xander starts at SI as Tony's heir

Things are going really well, they live happily ever after etc.

Loki somehow turns up, emotionally damaged, not trusting etc.

Xander meets him randomly and they banter again. (Xander decides not to turn him into SHIELD)

An odd friendship forms between Xander and Loki and, eventually, Cordelia.

And so, Loki is healed and reformed, and helps the Avengers defeat Thanos.

(Also, Xander and Cordelia get married at some point, and have children, and really do live happily ever after!)

Anyone is welcome to write any/all of these stories OR a spin off in their own direction. My only two requests is that you keep the main pairings the same: Xander/Cordelia and Tony/Pepper, and don't make them explicit.