I would quite like to know if Spigot are going to continue there forks of server mods (like CraftBukkit) because Spigot improved my server lots.
Spigot is very up in the air at the moment. Right now they are creating binary patches to the latest 1.7.10 version they released so they can update it to 1.8. Whether they will venture past protocol updates and try for a full 1.8 update or if they will eventually do something with sponge like they did with bukkit when sponge is old enough is all TBD.
I don’t know if md_5 and Spigot’s team will migrate Bungee to Sponge …
Bungeecord will support sponge because all it is is a proxy. I think the sponge devs are going to let sponge support the IP / UUID forwarding too. Really no migration will be necessary.
Spigot will be updating their builds for a couple of more months. If by then they haven’t obtained rights to publicly publish the software or source code, it will be for the best that you migrate to sponge.
Its not about Bungee migrateing to sponge but about it supporting Forge(the platform that sponge is built on top of) which is comming soon pending a [PR] : Add support for Forge 1.7 clients & servers. by dualspiral · Pull Request #1215 · SpigotMC/BungeeCord · GitHub made by @dualspiral