I dont think the player base will be split, over the last few months the playerbase has dropped, I think we can all agree on that.
Pvp is a popular way people like to play, so implenting a game type with the sole intention of pvp in the main game would be hard to do, (not impossible, but difficult).
So a game mode like arena makes sense, new faction, earn exp(repution by impressing) will get you to a max level of 50, you need 150 to hit pirate legend, do you see where I am going.....
New players to the game will have to go into adventure mode to become pirate legend, the new game mode will also bring life back into the game for pvpers.
I honestly think something along these lines will be implemented, this will strengthen the core game, give players that have left due to not enough action something to come back for and a new playerbase.
All positive things, it also means that if you hate one faction you dont have to do it just max out three of the four factions this will leave pvers not having to compete and pvpers having to do things in adventure, still given you the risk of you getting sunk and losing your loot.
Win all around and yes everything you earn in arena gets carried over because a cutlass you earn in arena doesn't have +5 damage compared to adventure as everything is all cosmetic.