I’m thinking SG could offer 1 of these per year (say, by completing Xmas event on 5*). Would allow players to reset one ascended hero back to 3-1 but you would get your 3* and 4* final ascension mats back.
Only available once per year and maybe used as compensation in extreme cases like the Telluria/Vela nerf. Compensate everyone who fully leveled these heroes with 1 reset ascension token and those who emblemed 1 reset emblems token.
Or some variation like this. Maybe you get 4/6 4* mats back.
Note that I would limit compensation to 1 ascension token for Telluria and 1 for Vela per account.