In general, I would be looking at summoning cards as something to use to populate an alternate Monster Manual, not as a alternate spell list. I also think there's a big difference between "I want to play the MtG setting using D&D as the engine" and "I want to play D&D using the MtG color wheel as an inspiration for a new magic system." As someone who likes the aesthetics of MtG but hasn't played it in decades, I find the latter far more compelling than the former.While I do think summoning spells would be important for an MTG RPG, I don’t think they would necessarily need to be super prominent. Summoning creatures is a big part of the gameplay, but in the novels, comics, etc. it’s not really that much of a thing. It happens - Garruk likes to summon beasts, Leliana raises undead minions, etc. but nothing beyond what you I might expect from a druid or a necromancer in D&D. Players in a game of magic are more comparable to pre-Mending Planeswalkers, who were practically gods, but the post-mending walkers who are the main characters of the storyline don’t really fight with armies of summoned creatures. They’re much more like regular, albeit gifted, Mages who can also Planeswalk.