I am not an expert in the gaming field, but you can check Godot engine for 2D games, people say it is a really amazing tool. Of course you can choose Unity, but many people now prefer Godot for 2D games.

I mean, the Flash maybe not the best option for creating a game, but for Flash website era, I think those weird, funny, and highly interactive websites are really shining at the age, I miss that time!