Good morning everyone,
I recreated a new dedicated topic since the previous thread has been closed by admin:
So, as soon as I read that custom format(s) feature implementation was available and thus the possibility to recreate a “Base-Set” only game mode, I immediately did it by following what simply suggested by admin himself:
So, after quickly creating this very simple game format (it’s a regular Base-Fossil parent with a selection query limiting the card pool to Base-Set ones), wanting to try hosting a new game under “Custom Formats” I correctly see the newly created and published game mode, but… What was not written in the closing thread is that this new feature (implementing and creating new games with custom formats like Base-Set only, for example) is available behind a paywall 
@admin if things change and the feature becomes available for free (especially in this early phase of development, where further testing of the whole process maybe is useful also to you),
I’d be happy to contribute by sharing with the community this and other custom formats I myself developed (“PROP-2”).
Cheers,
LorMus