If you're going to need this amount of storage, you're better off with cloud storage. Just open an account on Mega and upload some of your data there, that should give you 50 GB more than you currently have with no obligation what-so-ever.
But if you want a serious mobile gaming machine, nothing you want can be gotten at the size you're asking for, we're still not there - even for Gamecube. But, that's not saying it isn't possible - seeing as streaming is maturing, you can pair a device like a GPD XD with a gaming laptop that can be in your backpack or somewhere else. That will allow you both greater portability and power, but will be very expensive, and not as portable as just a handheld.