I feel like all the players that use slimefun consistently have completed everything they wanted to/are able to within the current limits of the plugin. To solve this, I suggest adding/readding some features to make the plugin more enjoyable and give players who have made everything a chance to do something new.

The first thing that comes to mind is the readdition of the nuclear reactor. This could be nerfed in terms of fuel output (eg half or 1/3rd of the base value in the plugin) but I believe it would be a good addition to the server. As far as I'm aware, no issues were caused in the N2 beta test when the reactor was a feature. Also, as we've now reached over a year of N2 (woo!) it's probably time to shift from a WW1 theme to a WW2 theme, which would allow for more mechanics and features in the server anyway.

(if you can think of other things to add, leave a comment!)