I doubt the girls will want to reform as a band without Sarah. Music wise, they’ll have tonnes of her vocals stored, unused verses, melodies etc. Sarah did solo work with Xenomania aswel so they could make new music with or without Sarah if they wanted to.
I think the best we can hope for is a celebration rerelease of each era and album with rarities like the Spice Girls. Maybe a rerelease of each single also. Sound Of The Underground album was released in May 2003. So it’s 20 years old in 7 months. They may be working on something for that. If it’s still too soon I think they’ll consider 21st Birthday rerelease celebrations instead.
I can’t see them ever touring again.
I hope to see new music via rereleases with demos and unreleased tracks.