I would think that a traditional fighting game would be easier, but it depends upon what you want to make. Since a game such as Super Smash Brothers has a good deal of content to it, let's stick to the classics.
You have games such as Street Fighter, Mortal Kombat, and Tekken, which have characters with individual personalities, moves, and move inputs. Do you wish to go this route? If so, you'll need to plan out their mechanics. How will the game handle the move inputs? How will it handle the hit detection? How will the game handle cool down for the moves? What about a combo system? These are a few questions you need to ask yourself.
Then there are the characters. How individual will they be? Is there some story behind all of this? What about their personalities? What will their movesets look like (if you're going the 2D sprite route)?
Once you've answered some of these questions, you can then answer other questions, such as how much health is necessary for each character.
Also, you'll need to plan out how this will work in the Godot Engine. So you may use a
RigidBody2D for the sprites, and highly customize the hit detection for them in the editor.
I hope I have pointed you in the right direction.