In my opinion, all the maps that were created made every server unique. There were servers like: build on a planet, freebuild in slate, clan build servers, DM's in the slopes or many other terrain maps (including GSF San Andreas). This was addicted for most of the people when they were out of fun, they would simple visit another server and make fun there again.
Today when I visit Blockland, a lot of servers are empty and there aren't many people online. Most of the servers I discovered are mostly identical to others, nothing special too.
Something that might make Blockland big again is to re-release a second client on Blockland's site, using version 16 and another name - Blockland Classic. Look to RuneScape, they brought back an old version of RuneScape, and the user base kinda exploded to a new height again. I don't know why Badspot shouldn't try this out, it may be worth it.. even if it was just for a year to see if people really play the old version. I would play for sure, and release my new interior maps I made in the past three years for a Vanilla mod called Blockland Classic Mod.
Here is a map I'm currently working on in v16, these are old screenshots.