Excited about the project

[NiLSPACE]

Tool repairing should work, we had a Fixies plugin for this, and I believe it was later merged into the Core.

Yes it should be implented in the Core.

On the other hand, it is currently impossible to write a plugin that would provide Nether portals, simply because there's no way for the plugin to create the portal in the opposite world (potentially unloaded chunks)

It could be done. Mark the player who just went through a portal, and when he spawned create the portal.