I would love try routing a 100% secrets category, but can we have that category in this game or it would be just like 100% or something and not only 100% secrets + Any%? And can we actually have that category or not?

In DOOM there's a category called UV-Max which includes getting all secrets as well as killing all monsters. With Heretic being DOOM engine and keeping track of both secrets and monster counts I think we could have some sort of a Smite-Meister Max (SM-Max) if you will.

Thats would be kinda interesting to route even with monsters, im already wanna start thinking of path to get this as fast as possible, but why only on Smite-Meister though? Can this run be on any difficulty?

I guess you can use whatever difficulty you like, but Ultra Violence is the standard difficulty setting for DOOM speedruns, so I think it would make sense to do the same for Heretic, with Smite-Meister being the equivalent.