I believe the S series are a break from the main plot, though. Many, if not all, are side stories that aren't moving the plot. Some are flashbacks, some are back stories, others just short stories, etc. SIII will most likely compile those five, especially since they all were released after SII.
S sounds cooler than Gaiden: SnSS. See? Two S.
I'm wondering again, since Kaguyahime no Shana and Ojousama no Shana exists, and most likely won't be complied in SIII... are there any other short/side stories that S and SII didn't collect? (Asking Elf) Just how many short/side stories are there total?
Also, for fun because speculating is fun. Two categories: most powerful, and most skilled.
Top 7 Flame Hazes (any time period, including deceased)
Top 7 Flame Hazes (current and up to the latest volume, excluding deceased)
Top 7 Flame Hazes Jizaiho Casters (any time period, including deceased)
Top 7 Flame Hazes Jizaiho Casters (current and up to the latest volume, excluding deceased)
Same as the above for all Crimson Denizens.