Open up the starter sector blocks of the opposing team. When entering them, you are a) beset upon by massive PvE resistance and b) flagged for PvP.
Then add an ability to voluntarily flag yourself for PvP.
Lastly, make the larger stations' NPCs massively difficult world-boss types. Allow Klingons who dare to traverse Sirius block (and vice versa) and try to take the Admiral and Sulu down, facing not only escalating PvE forces, but becoming PvP flagged themselves, and as such, facing any Federation player who chooses to flag themselves for open-world PvP and help repel them (both in space and at the stations).
I hate (err.. strongly dislike) your first suggestion. That was the end game available in SWG at the beginning basically.. PvP in Bestine. It was fun, but hardly interesting enough to be called end game. End game should never be "choose your own PvP". They need raid content (5-,10-man,Fleet) and good itemization.