I just got a brand-new edition of COTW, and I'm stunned at the number of new cartridges that have been developed. (My old copy is about 20 years old.) Aside from the proliferation of JDJ and Lazzeroni cartridges, I was particularly surprised by developments at the "itty bitty" and "shoulder cannon" ends of the spectrum.
On the "pipsqueak" end you got Eichelberger making .10 caliber cartridges by necking down .22 LR - seems like a helluva lot of work to end up with a cartridge for taking field mice at extreme range. On the "Good Gawd!" end, you got a 20mm brass shortened and necked down to .50 caliber for 1000+ meter shooting. I think that Anzio Ironworks pioneered that with their 20/50 Anzio. Then there's the .729 Jongmans - sounds like an idea inspired by Ed Hubel's "12 Gauge from Hell".
That .950 JDJ has got to be the wildcat to end all wildcats. What someone is gonna hunt with that artillery, I don't know, unless it's armored vehicles. (I've heard they taste like chicken)... Seriously, even if you were going to hunt something with a 100+ lb rifle, it would have to be mounted on a jeep or something. I imagine that the few rifles made for that chunk are purely range queens.
And there are lots of "normal" sized cartridges that were new to me. You would think that with a couple thousand different cartridges in existence, there wouldn't be any need or room for any more wildcats. But there is always some bright wildcatter who perceives a need, and sets about making a cartridge for it.
Heck, I even invented my own wildcat - the .460 SB Magnum. It's really just a rimless .460 S&W made by shortening a .30-06 case and expanding the neck to take a 300 gr .452 bullet. I shoot it in a rifle built on a CZ24 action and a Rhineland .45 ACP barrel with the chamber lengthened. It's intended to provide the shade-tree gunsmith a powerful hog rifle that he can build fairly easily. Currently I get around 2000 fps out of a carbine-length barrel, ginning up about 2600 ft/lb ME. I'm working up towards 2200 MV if I can get there without pushing the envelope too much. I don't foresee it becoming a world-shaker, but it's been a fun project!