Doom is a really interesting piece of casting.
On one hand I'd like to see a more seasoned older actor with the requisite amount of gravitas and measured menace. On the other hand a younger, more deranged, unbalanced Doom would be perfect for some of the superb new talent we have knocking about at the moment.
Maybe even someone who could successfully straddle both groups like the excellent Sam Claflin. He'd absolutely smash it out the park imo.