This app was mentioned in 9 comments, with an average of 22.33 upvotes
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Another free to play game with that is also based off a larger gaming property is Trials Frontier. They also didn't have a PvP mechanic but then later added one. This would make a good case study for Extra Credits' theory.
You will only need Trials Frontier.
No p2w and no ads. Easy to learn hard to master.