Mageia is nice as an alternative for Fedora/OpenSuSE. There is nothing like a professional RPM distro to have the best of both worlds. Everything just works, and is easy to configure.
Fedora is nice because it has everything, but it's not nice because RedHat is constantly doing stuff that shows that they have a hidden agenda. They're constantly messing with the repos etc. Besides that, it's still arguably the best desktop distro for productivity, because everything just works immediately, it's the most bleeding edge but also the most stable, and it's clean and very practical.
Debian Testing is an alternative if you don't mind solving all the problems other major distros solved more than a decade ago to get started. After the initial time consuming setup, Debian doesn't have much updates besides security updates, so it's pretty easy going. But it is time consuming and nerve wrecking to get going. Ubuntu takes that time consuming initial setup away, but Ubuntu just doesn't work, I've tried, as I always do, and Ubuntu Core 14.04 is pretty decent, but Canonical's Ubuntu is just a complete mess, which is sad really, because if you would have asked the same question before Ubuntu 12.04, I would have most certainly answered Ubuntu. Kubuntu is pretty nice, but it lacks that productivity focus. Xubuntu is probably the most productive Ubuntu Core based distro, and arguably the nicest. Mint is also a complete mess, Cinnamon is going nowhere apparently, and where the community distros of Ubuntu iron out Canonical's bugs, Mint seems to make them worse. Pretty useless distro.
Gentoo is a commitment, if you want a productivity focus, it may not be the most evident choice, although if you go binary on it, it also has the equivalent of delta RPM for efficiency.
I'd stick with Manjaro if I were you. It really has everything for a non-enterprise gaming user, it's based on Arch, which has the most software available for desktop use and server use, it's fast, and it's not US based, which is becoming an ever stronger advantage these days.