TDM... Unlike arena its 1 fort, where you can practise PvP. Maybe even different "maps" like a smaller version of the SoG island. There would be no complaining sailors that they have been sank for "no reason". There would be no cosmetics so its entirely PvP focused and would likely be rarely updated. With the recent removal of Arena, Adventure has been full of whining players who cry when they are sank because they like their gold. This mode would be loved by the TDM community and would likely engage newer players to come practise their PvP skills.
Rare previously stated that less than 2% of the player base played arena, with this in mind the TDM mode would have a significantly smaller amount of servers than arena and would require less memory because the maps would be smaller.

