Magellan isn't a Marine, he's a World Government Chief Warden.
Being an Admiral isn't just about powers and such; you need to be familiar with the whole idea of the Marines. Being the chief-warden in a prison is very different to being an Admiral.
I still can't understand why people want Magellan. I see that he's more popular than the VAs generally, but still, him being an Admiral seems illogical to me.
Well there's several reasons.
First, the Marine is part of the World Government, i don't see a reason why Magellan couldn't be made a marine and "be familiar with the whole idea of Marines" in 2 yrs. Anyway, is job is mostly dealing with pirates, in colaboration with the Marines, it's not like he's newbie on Marine's stuff.
i don't think Magellan will want to continue as chief-warden, and will instead be chasing those lvl 6 people that ran away from Impel Down, and also as i already said, Magellan is the strongest contender in "power level" that was shown.
Either way, this is just speculation, but his character name is after a known portuguese navigator known as Fernão Magalhães, which is commonly translated as Ferdinand Magellan. It's weird Oda choosing such a name to the character and not making him be a "marine" or somekind of.
Also, i think Luffy might want to revenge Bon Clay :P