The Community believes that the back-end specifics unique to Arena make it incompatible with Casual, so, I've recreated Arena using VScript in hopes that it could help Valve to bring Arena to Casual, as this is something the Community would like to see happen.
This hope is supported by arena_lumberyard_event and arena_perks (the two maps also recreating Arena from scratch) being featured in Scream Fortress events.

It's designed to be almost a drag-n-drop replacement, with no changes to the game's code, and with minimal changes to the maps. The script reroutes entity I/O for the stock maps, so, Valve wouldn't need to make any I/O-related edits.

This is a faithful recreation of Stock Arena with just 1 change: it uses the mini-round system found on arena_perks and (formerly) arena_lumberyard_event. The hope is that this'd let Valve avoid using the community gamemode toggle used by Casual for VSH and ZI, as the toggle has the history of being quite unstable. This also helps the Community Servers, as they won't have to create a separate round limit config just for Arena, and they could simply add them to their map rotation. This is, technically, Player Destruction where a "full round" lasts for around 10 minutes.

Arena VScript Port Maps: https://steamoss.com/sharedfiles/filedetails/?id=3360196477
Download the mapper's version of the Script: https://tf2maps.net/downloads/arena-vscript.18119/

1. Delete the old gamemode-related logic entities, except for `team_control_point_master`.
2. Create a `logic_script` with "Entity Script" set to "workshop/arena.nut".
3. A compiled map must include files `scripts/vscripts/workshop/arena.nut` and `resource/workshop/hudarenavscript.res`.
3.1 If you don't see the Arena HUD, chances are the res file is not packed into the map.

Two notable exceptions which required extra modifications are Byre and Sawmill.
With Byre I was given access to the original and up-to-date source files, so, Valve can simply use those.
For Sawmill, I had to add setup gates and respawn room triggers.
This version of arena requires those, and all maps except Sawmill already have them in place.
I've also made a separate vmf just with the additional entities.