The most popular ones are projects aiming to recreate Fallout 3 and New Vegas in the Fallout 4 engine, for obvious reasons. They look to be coming along quite well all things considered, although due to legal obligations, I'd imagine that there will be unofficial mods for those mods to import the old voice acting back, seeing as the mod teams are not allowed to use the old voice acting from their original game(s).
Fallout: Cascadia is another project for Fallout 4 that seems to be coming along nicely.
For New Vegas, The Frontier is also being worked on and developed currently. Honestly, my thoughts are that they should develop this project in the Fallout 4 engine just so it doesn't get outdated as fast, but I understand that it would be immensely problematic to probably have to start from scratch after all that work.