I think Revised Towers Standard, Open and Pre-shadows Multipath already give quite a few options in this regard, both for friends with fixed decks and just random players (Do people bother with Fruit Loops in Revised Towers Standard?).
Revised Towers Standard makes it sound like it's basically a modified version of Towers Standard, which it is not. It's literally a format for testing things for TS Sealed. It's the closest thing we have to a "freestyle," but the site limitations do limit the possibility for testing new format ideas. In many ways, the site path defines the format.
Open is a legal format, not really a testing environment.
Pre-Shadows Multipath is a specific format, one which I (and probably many others) don't particularly understand. So, it's fine for those who like Pre-Shadows Multipath, but for those who want to test out new house rules, it doesn't really fit the bill.
but if someone were to do this, I certainly wouldn't complain. Sites would probably give the most trouble, both coding and rules-wise.
Pre-Shadows Multipath says "Sites from block: Special." How do the sites work for that format? That one's got weird site rules, might give us a hint as to how things would need to be coded for a true "freestyle" format.
Interestingly though, Revised Towers Standard and Pre-shadows Multipath are rarely played, despite the various options they give, so I wonder if such a format would really be popular?
Players gravitate towards what they understand, that being popular legal formats. RTS was (as far as I know) only made for playtesting purposes, so it never really felt right for me to try to play it, if I'm not actually testing anything.
It might be interesting to have a no-holds-barred TSR constructed league, though! That would really encourage people to take a closer look at the possibilities with that format.