NanoTech Future
Xander dresses as a scientist from a future where Humanity mastered NanoTech. All humans are nano-augmented, any human can terraform a suitable planet, moon, or asteroid. Humanity is alone in this future with no alien life in the galaxy so they can explore and expand at their leisure.
The Powers permanently banish Xander due to the power the nano-augmentation.
Xander lands in the StarTrek verse in Borg territory. He takes over the collective when they try to assimilate him. He then works to return the Borg back into individuals with just some extra nano upgrades add on to their person.
Xander leaves after he finishes with the borg and takes those who wish to with him for new adventures. Borg Tech acquired. TrasWarp drive for FTL.
He eventually settles down in a mix verse of Babylon5/StarTrek/BSG 2003. He makes contact with the Final Five after the end of the first Cylon war and helps them (no cavil model messing things up) make peace with the 12 Colonies(The 12 Colonies and the Cylons are in the Pegasus Dwarf Galaxy that is really close to the Milky-Way) . 500 years after making a Government with them. They make first contact with Earth before the Vulcans. Earth and the later the Federation's attitude towards augmentation is relaxed because it was found out that the first-gens during the Eugenics wars were mentally unstable because the scientist didn't know what they were doing, No Romulan empire, No Borg Collective, No temporal cold war, Federation includes the Romulan and Klingons, No contact with the Mimbari until Kirk era. Kirk is still Admiral (the movies didn't happen), Babylon 5 is still built.
Verse:Home(Buffy), Costume(? future), Vist1(StarTrek), V2(Prototype), V3(BSG 2003), V4(Halo), V5(MassEffect), V6(Andromeda), V7(Babylon5), V8(StarWars), V9(Stargate), NewHome(B5/StarTrek/BSG2003)
Prototype verse: He collects a sample of the BlackLight virus Alex Mercer so he can adapt it to himself and the Ex-Borg that left with him.
BSG2003 verse: He adds new ship hull types to his fleet to replace the geometric design of the Borg, and to get new cloning tech. (BattleStar ships and Cylon resurrection tech).
Halo verse: He get copies of the Spartan II enhancements, UNSC tech, Forerunner tech.
MassEffect: He gets copies of MassEffect tech for sub-light maneuverability and agility, and weapons. Also HardSuits (BloodDragon)
Andromeda: He gets copies of AI tech and Andromeda for ship hull types.
Babylon5 verse: He gets copies of Vorlon, Shadow, and Mimbari tech, and discovers a new place to hide his growing fleet. (white-star, Excalibur and jump-space)
StarWars verse: He gets copies of their tech.
StarGate Verse: He gets a copy of the Ancient database in Atlantis.
Each of the Verses he goes to he gets copies of their tech to add to his tech database.