1. Less control: have them attack at disadvantage
Everyone has mentioned rolling with disadvantage and it makes a lot of sense especially when you consider the video from Icyfire’s answer demonstrating how a trained swordsman struggles when trying to use a two-handed weapon with one hand.
2. Less power: lower the dice by one type
There are several weapons that already have the versatile property, which means they’re supposed to be able to be wielded one-handed. These weapons still go down a dice type when you do this, though, to reflect the reduced power you have behind them.
3. Optional: Have pre-requisites
The thing with adding disadvantage alone is that, for instance, a gnome could wield a greataxe in the dark with just one hand, with another hand free for anything else, and despite the small size and lack of proficiency and everything else the only penalty they’d face is disadvantage. Have them successfully hide and it’s a straight roll!
Perhaps you could have a minimum strength requirement: only a character with a strength score above 15 can actually wield a two handed weapon with one hand; or maybe you homebrew a feat that allows this. In that case you might not even need to add all the other penalties...
4. Optional: Lose/alter the proficiency bonus
Losing the proficiency bonus entirely is a pretty intense penalty, especially at higher levels. It seems reasonable that if someone is proficient with a greatsword they’d be passable, at least, when using it one-handed. However, if you try with the other two elements and it still feels unbalanced, you could halve the proficiency bonus, for instance (rounding down as you do with everything in 5e.)