Not necessarily true. After all, most Mega Evos are from Pokemon that wouldn't evolve further anyway (fully evolved Pokemon from a 3 stage family and a legendary) and some fully evolved Pokemon from a 2 stage family that already had at least okay-ish stats.
Personally, I think anything that can't learn Hyper Beam/Giga Impact and doesn't have a Mega already can potentially evolve further (and some Pokemon that are currently considered fully evolved too, though it's less likely).
This may seem crazy but what if you link trade a Zangoose and a Seviper , Zangoose and Seviper evolve. Sort of like shelmet evolves only when traded with a karrablast --- FC - 5112-4767-5140, FS - I need to find out Lapras>Everything...Except for Scizor, Charizard, Jolteon, Glaceon