@DalBrar contacted me with a visual border implementation, which I think looks nice.
https://github.com/DalBrar/WorldBorder-Particles

It currently only properly supports rectangular border shapes, not round/elliptic, and lacks config options outside of directly editing his Particles.java, but I figured some people will be interested in building it for themselves anyway.

Here's what his implementation looks like:

I like it.

I'd toyed around with using particles to visually mark the border edge in the past (also the possibility of using phantom blocks sent to the client which don't really exist on the server), but my efforts didn't turn out very well and seemed like they might particularly affect client performance, so I'd scrapped the idea. Based on the example he provided above, I'm revisiting the idea, even though I'm generally not actively developing the plugin currently and am rather busy lately.

To properly implement this, I think I would need to implement an additional "inner" border with a slightly smaller radius based on the actual full-sized border, which would be checked initially instead of the actual border to detect players nearing the actual border (so as to only require additional checks on people near the border), then a reasonably clever method to determine and possibly cache the X/Z locations of an elliptic border's boundaries near the player for display. Display of rectangular borders is probably covered well enough by the code above, though determining whether they need to be displayed could be better handled in the new "inner" border checking code for less processing overhead.

Hmm. Not sure when/if I'll get around to it, but his code has made me somewhat interested in trying to more fully implement this for a visual border.

@trollogyadherent Sorry for the late reply, yes I still have the code that was suggested as the original idea. Hasnt been touched in an extremely long time, do with it what you'd like.

MC-WorldBorder-Particles-master.zip
.

I love the concept and use of the barrier block for a world border. It makes perfect sense to use it when the world border is established. I would love it if this came with configuration options to vary server-by-server:

• Allow for showing or hiding the barrier/border. We know barrier blocks can be seen in creative mode, but not in survival mode. So having something like - barrierBorderVisible: true/false would be great.
• Allow for selection of using barrier block or bedrock. This would allow server-owners to make the other side of the world visible or not.
• Then add settings to allow for breaking the barrier/bedrock.
  • bedrockBorderBreakable: true/false and - barrierBorderBreakable: true/false
• Allow for either block used to allow custom textures to override its visual appearance

I've seen some configuration files that allow for breaking bedrock or not, so having an option to turn it off for the border would make sense for both barrier and bedrock blocks. At least this would be a nice enhancement to it later. :-)