Full Endgame systems haven’t been implemented yet. Monoliths still have features planned that aren’t implemented, and I highly recommend taking a look at this page for planned end game systems: https://lastepochgame.com/end-game-systems/
Arena also wasn’t planned as an endgame system, it was implemented as a filler and has been “surprisingly popular” according to the devs. What we currently have is incredibly basic system-wise and if the devs decide to keep it we should expect iterations on it in order to improve enjoyability, playability, and sustainment as an endgame system.
Please remember that Last Epoch is in Phase 2 of the planned development roadmap as shown here: Early Access Development Forecast. There are still many missing portions of the game, and we can expect a large amount of content to be developed between now and the current Q4 2020 release date.