It was recently revealed the contents of Kamen Rider Saber Blu-ray COLLECTION 2 which will be released on August 4. Among the contents it was listed that there will be an Original Spin-off content included. It's not known it is focused on which Rider. But considering that there is no Spin-off in COLLECTION 1, this means the character hasn't appeared in the TV Series yet.
This Spin-off could be for one of the remaining two Riders who are yet to appear in Kamen Rider Saber. One of the new Rider has to appear between Episode 25 to 32 to make it up to Blu-ray COLLECTION 2 which contains Episodes till 32.