The power restriction is tough, but red has plenty ways to make Loki large enough to copy whoppers like Mana Geyser
Mondrak is an obvious fit into a Karador, Ghost Chieftain deck, especially one that likes to gain life (like my Halloween with Karador does) in order to offset the life loss from spending Phyrexian mana
Letting it have flash means you can sneak it onto the battlefield when one of these cards is on the stack, but I dont think youre going to need to be so tricky with it
Story-wise, planeswalkers can die, but between rebirths, origin stories, igniting and losing their spark, there is plenty of space for your favorite character if R&D deems it so
There are a few select commanders that actually have mutate, so I also included a few non-human legendaries that are great mutate targets that enable powerful shenanigans
Youre rewarded not just for stuffing your deck full of black-green squirrels, for example, but also for foraging with food tokens and graveyards