Because you can in theory go after the Sage of Spirit at any time, I'm going to propose the potential of Mech%, or 'Gain the Sage of Spirit as fast as you can'

The objective of the run is, of course, to clear the Spirit Temple as fast as you can, which can prove to be a possibly interesting challenge, as you need to obtain enough hearts to pass through the health check guarding the head. So routing would need to find a way to get the six Heart Containers you need after the prologue as fast as you can.

And another idea I had is similar to one I believe Breath of the Wild speedrunning had, which is of course getting the Master Sword as quickly as you can. However! You have to do it the legit way, in that you need the two full stamina wheels to pull it out from the Dragon of Light, and not the way that Any% gets it for the final fight. Also, you can't do it through item transfering from a prologue save, as that's a different Master Sword.

For Mech%, it sounds like another GSI + one more thing category, however, there likely will be an IL of the spirit temple so that can be run on its own. You could also just run AD or AMQ since that beats it but I doubt there'll be a run that exclusively beats one dungeon.

As for MS speedruns, it will be one of the categories added at a later date, currently the ruleset is being figured out but it will be added when its ready.