Just idea that came to me as looking at requests for Sage Mode, plus this phrase on the SL Wiki under CS
Curse Seal on SL
There are currently 2 stages, each is related to the stage in the original series but different in how you get and what you can do with it.
Note that also there are currently no subtypes, but this might change.
So what if we redo them both, make the SST Sage Mode, ie SST could be the first stage like Ero Sannin was at, and when you train to get the full version that is the SSTE.
Each step could have certain skills attached which relate to Sage Mode as well.
The Same with the CS, have the different subtypes which could have different effects in relation to them, and for CS2 have skills related to that.
Just my thoughts on the subject.