Gdevelops platformer behavior is already better than most default platformer templates I have seen in other engines, but with these 2 improvements I think it would be perfect (and actually usable as-is for any platformer game).
Add an option for jump/ledge forgiveness. Basically allow the player to jump in midair if he touched ground in the last X frames. Pretty much every platformer wants this.
Let us define the jump with 2 mathematical functions instead of just parameters. One for the jump and one for falling. That way it would be much easier to have the character move exactly as you want it to.