In response to SMP seeing less popularity, I think it would be a good idea to utilize Towny as a focal point of the server to start to encourage players to play again. One of the primary functions of Towny since it was first release was as an aid to PVP, but it was also intended to be a plugin to build your server around. Allowing people to enable and disable pvp is unnecessary because Towny has PVP protections built into it. If people don't like pvp, they can join a town with pvp disabled or a town that is peaceful with other towns, or a town that has built their defenses to prevent it. If they do like pvp, it adds a layer. Towny without pvp is mostly purposeless, which is why in less than a week I was able to build the most well-established town on the server (even if we didn't have the most town farms) with the most people and the most usable space. However, once we hit that point, towny became pointless. PVP gives players a chance to solve problems with swords and not words (thus clearing up your chat). Even if you had to get rid of CE to encourage a more fair and balanced PVP experience, it could provide a deeper experience. Rather than grinding for lvls, people would be waging wars, fortifying their bases, etc. Personally, after a week of getting crap rolls on CE I gave up and then realized I was out of stuff to do. Dungeons are fun, but one dungeon every 4 hours isn't enough to keep players playing in a game where projects are crucial, dungeons aren't really a project.
To add to the projects idea, a more efficient antilag plugin would be amazing. The fact that redstone creations are heavily limited by the antilag plugin means that any late game building ideas that I would try to build are less feasible or even impossible unless I'm just building another building. Using this antilag plugin essentially instantly dissuades an entire demographic of Minecraft players from stayin in the server.
I appreciate you taking the time to read my suggestions, hopefully I can be of some help :)