Eternal formats are generally not "thought up" by WotC. Commander, Pauper, Canadian Highlander - these are all fan produced formats. Even Vintage and Legacy are not formats per se that Wizards has concocted. Rather they are defunct formats (renamed) that fans have held on to. Wizards has shown no interest in supporting formats that utilize reserve list cards. I think that is the real reason that Wizards does not support eternal formats.
I have thought that a new constructed format would be needed. Ever since the debut of color-coded expansion symbols (which was Exodus in 1998), I have thought that rarity should play a part in deck construction. Something in between Pauper/Peasant and typical constructed.
Something like allowing a maximum of 2 copies of any rare, 3 copies of an uncommon, and 4 copies (maybe 5?) of a common.
Or something like a deck must have a minimum of 20 common cards (not counting basics), and 10 uncommon cards.
Happy Brewing