Mcsomething it had to be. The question is what is the something. McVeggie doesn't work, because veggie burgers became synonymous with the last generation of plant based burgers. McPlant does sound awkward right now but with the full backing of mcdonalds marketing / their resturant footprint it will roll of the tongue within a few months of launch.
I'm just throwing this out there, but maybe there was an equivalent to what "Supersize" meant for going larger in size instead that's geared towards plant-based versions of each burger. So maybe I still want a Big Mac, but I want it with these plant-based burgers but still with the extra slice of bread and the special sauce.
I don't have a name for this as clever as Supersize, but it seems feasible.
If they wanted to get sued for trademark infringement.
(McDonalds is working with Beyond Meat, one of Impossible Foods' largest competitors, for their plant-based meat substitutes; deriving their product name from a competitor's product is almost certainly a bad idea.)