I think they should end the idea of merging D&D and M:tG - and instead build an RPG system designed to work with the concepts of M:tG rather than shoehorn it in where the mechanics of the D&D system do not serve the M:tG Lore. D&D, for example, does not handle multiple summons right now - and that is essential to doing M:tG justice. If they do that, I'd say start at the beginning Dominaria, as the core setting.
The MtG stories accompanying the sets don't really do anything about the MtG mechanics and summoning either though, does it? It feels like D&D couldn't approximate the cards, but would do a better job of approximating the stories.