I don't think an overarching plot is necessary, per-say, however making sure there are things that are happening, and that those things can affect most of the players involved, is crucial. Instead of having a traditional plot where a group of heroes save the universe (yadda yadda), we need [b]groups[/b] of heroes doing what they themselves want to do. Thankfully Star Wars helps with this considerably, as there are many existing factions which can help players interact, namely the Jedi, Sith, and Bounty Hunters. 
Said groups can have an overarching goal. I imagine Sith vs Jedi being a good start, as an intergalactic war is a surefire way to make sure something is happening most of the time. That's not to say there can't be downtime, but it means both the players and the moderators can create their own scenarios with relative ease [i]and[/i] get other people interested. So a group of Jedi might be taking a mission on Planet X, another on Planet Y. The Sith may also have a group on Planet Y, but also on Planet Z. Depending on what these groups want out of the experience, they can be shifted around to be grouped or isolated. Maybe two groups want to do combat, they can both go to Planet Y. Another group wants to do an assassination mission, but is open to interaction with other players, so they can also be put on Planet Y. 

Of course this will take a lot of organisational skill to run, but having multiple reliable mods can help ease that burden. Apparently disappearing mods was quite problematic for Expanding Horizons, so it'll be a good idea to make sure the current group is ready in advance. I'd recommend either a dedicated posting time (once a week?), [i]or[/i] have both the players and mods put a tl;dr paragraph at the bottom detailing their post, so people with less time can focus on writing rather than reading. I prefer this second option, personally, because it'll mean the flow of the RP can continue and new players won't have to read through pages and pages of text just to get caught up. I'd also recommend creating a "database" thread of sorts, detailing everything that has happened in the RP so people can catch up when they need to. This will be much more important for a Star Wars persistent world, because there will be things that happen which can drastically alter the universe. I imagine a dedicated mod taking this position, and individual moderators PMing them the details of, for example, a week's worth of posts, which they then re-write in an order which makes sense. 

Lastly, for the love of god, players need to understand that they will be taking their narrative into their own hands. This is something that confused me in Expanding Horizons when I attempted to join, and I'm sure it made others a little confused, too. The groups that form need to understand they'll be working together to make a story, and should voice their opinions on what they want so nothing collapses halfway through. Not too sure if this was a problem for the groups in EH, but as someone who wanted something specific out of the experience, I left because I didn't speak up and ask around or create my own thread. Players need to know they're all adopting a semi-GM role here, and sometimes they'll have to write for themselves, unless they gain the interest of others. 

But yeah, that's basically it. Maybe try to streamline the CS too; I liked the power system attempting to create balance, but it was... big. Expansive lore can create balance all on its own, so as long as everyone is on the same page. Give a hard limit on power. If the groups can interact on top of this, then we should be golden.