There are three things that I feel would make the game a lot more successful and do so in a fairly rapid time frame if it's committed to.
What can be done right now
More PvE missions spread throughout the levels, not just end game content. It shouldn't be unreasonable to expect 5 or so missions for PvE missions a week for Federation and 10 or so a week for Klingons until the game has enough content. Think that's a lot? Well it's the number one issue with the game right now is a lack of content. Without this priority, I personally feel that the game won't live to see the long term. People buy the game and are driven away by BOREDOM.
What should be worked on in the background right now
Rework the game to get rid of Sector Space entirely. Instead have the Helm plot a course and have it take a certain number of minutes depending on distance to destination where you can sit and watch the stars go by, read up on tactical reports of what's coming up in the mission you're headed to if at all, or just press a button to skip the transit time entirely. I for one would sit there and wait through the transit time unless a team is waiting for me.
Combined with open teaming, this also would have has the added benefit of making instantiated zones not feel so instantiated. Instead they end up feeling a lot more sandboxish and allows for a much less jarring transition to sandbox content which a large portion of the player base feels is missing entirely from the game.
What should be done as soon as the new method of interstellar navigation is in place
Once Sector Space has been eliminated and the current mission content can be reached in the manner described in mid term, then large, expansive, sandbox zones can be added. Again, this would seem fairly seemlessly integrated.
It's something similar to CO, how you have your sandbox zones, then you have your caves and buildings and whatnot that seemlessly integrated into that.
But here's he clencher. Space is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.
What I'm saying is that due to the vastness of space, you can have untold numbers of sandbox zones to make the game feel extremely vast. Eve or Earth and Beyond anyone? And the zones can be absolutely huge. In Earth and Beyond for instance, you could go into warp for a couple minutes before ever seeing a group of asteroids or mobs. Mesh the two predecessor games together and you'll have a winning combination for sandbox content.
What's more, with interstellar travel working the way it does in my idea, you don't even have to worry about pinch points or traversing multiple zones. Just warp and go.
Heck, have the option for exploration missions and plot twists to randomly occur mid-transit.