I haven't tried out Zone yet and I haven't really looked into it too much. I'm basing my opinions on what I have read and my experience with what seems to keep people on Classic. I would suggest focusing on two areas:

1. Social Features

I knew that Zone, being a minigame-oriented server, was going to pale in comparison to the likes of Era and Classic. The reason for this is simply because both of these games have lots of social features and areas for players to fall back on when they simply want to do nothing. Everything that you develop should aim to strengthen a community. Often these features will encourage people to be lazy, but at least it keeps them on. Missions could potentially be a great social feature if you tailor them that way.

2. Competitive Features

Complimenting your socialites you will also have your hardcore, competitive players. These people want to be top of anything, they want recognition and they want to be the best. That's great, so offer them a large range of activities to do so. Squad vs Squad activities are a good start, I'd go here first. Then you can start introducing new planets and possible weekly or monthly tournaments for the current ones. I believe that tournaments are definitely the key here; regular events just seem to alienate people. Keep them wanting to win and they will stay on until they can. You'll need to keep on top of this community though because it can turn into a negtive mess.

Single player activities are very anti-social and I don't think you can really consider them at this stage in your development. Get them hooked on the community before you attempt to exploit them with things to buy.