Well, he was only the "strapping 30" in the second trilogy, which was supposed to take place in the late '60's. The original version of Magneto, as well as Xavier, looked more like late '50's to early '60's , which would still need to be explained by some sort of mutant longevity, just not as extreme as Logan, who fought in the American Civil War (A K A the War of Northern Aggression)
I think they should give other mutants a chance. At the minute, Wolverine is Hugh Jackman, and he did such a good that a replacement isn't going to be so positively received. There are other mutants they could do who didn't really get time to shine in the Fox movies such as Gambit, Rogue (she's practically a shoe in because of Carol Danvers already being in the MCU), Beast, etc.
Although Magneto is the father of Scarlet Witch and Quicksilver in the comics, I believe in AoU they mentioned their parents were dead (and they couldn't use the family connection due to Fox owning the rights to Magneto (as it was there were two Quicksilvers), however there is Polaris who is Magneto's other daughter. She has similar powers to him, therefore she could be used instead, thus ticking the PC box (female) but having the use of Magneto's powers without genderbending Magneto.
The X-Men series has a ton of mutants of varying race, gender, sexuality, religious beliefs, etc so they might choose to use some of the lesser known ones that won't be compared to the Fox movie versions. It worked for guardians of the Galaxy (bringing lesser known characters into the mainstream).
My guess is that they'd stay away from Magneto, Xavier, Wolverine, Jean Grey (and by extension Cyclops) because of the heavy coverage they got in the Fox movies. They've already covered the Phoenix saga twice in the Fox movies, the MCU doesn't need to do it a third time in the next few years.