Oh 2D. Then I think animation can be a good start if you are into this, like basic walk/run cycle for 2D sidescroller, jump, attacks and all that good stuff. Background arts is also great.
If you can do all of this, you can basically do a simple sidescroller by yourself, if you can code a bit or find someone 
You can also look into 2D isometric arts (environments, props and characters).
I feel like with 2D, you have 2 main options, sidescroller or isometric games so look for those games to get inspiration and to see where you can start and feel a void.
Overall, if you can draw anything in 2D, you are basically able to work on any 2D games or even 3D (if we're talking about character design, worldbuilding, concept arts etc), it's just a matter of specializing in specific department afterward.