Jacob

Eternal Flower Floette has been confirmed to not evolve based on the unused game data found in X and Y's files. In-universe, its path to evolution was cut off after being brought back to life using the life force of countless other Pokémon, giving it eternal life. It's essentially a legendary/mythical Pokémon, and since Mewtwo and Diancie can mega evolve, it stands to reason that this one can too.

Can Eelectross even get any more powerful? It has good stats and a good movepool, and its typing combined with its ability means it has no weaknesses. Also, yes, we have a Mega Gallade, a Mega Gardevoir, and a Mega Glalie, so we were overdue for a Mega Froslass.