MGS4 tied up most of the series loose ends or at least all of the ones that matter. Sure, Kojima copped out a lot (when in doubt, blame nanomachines, but it didn't turn out half bad considering the hole he dug himself into with Sons of Liberty. A reboot of the series makes more sense than a sequel though MGS5 is inevitable.
A long-gap sequel with Raiden as the badguy is kind of just biting off of Suikoden 3.
I'd personally go with a reboot or an alternative sequel. I'm curious what would happen if Konami stuck with the original concept for MGS2 which apparently involved Liquid Snake (real Liquid Snake, not fake Ocelot Liquid Snake) highjacking Metal Gear in the Middle East or something to that effect.