Seems to me there are several reasons we might not expect many (if any) new .20 caliber factory offerings. The .204 Ruger (along with .20 Practical 20/222, etc.) are fully capable of getting just about all the velocity one is probably going to want to run the 32 and 39 and 40 grain bullets at. These cartridges just plain excel over Vartarg and smaller cartridges for the .20. I see the guys necking down .22 Hornet and such - but that is like building separate .22 rimfire rifles for the short, long and long rifle. Why do it when the long rifle does it all just as well with a bit more powder. For a new cartridge to be commercially successful, it will have to either be far more effective or far more efficient. It doesn't make much sense to me to try to reinvent the wheel when one has velocity limited to the limitations of bullets.
That said, IF we had a larger selection of larger bullets for the .20 (both leaded and lead-free), a faster twist to spin them, AND with a new cartridge with powder enough to launch them at "small bullet" velocities, that would be something worth looking at, especially while maintaining the .223 bolt face and keeping the cartridge small enough to run in an A/R. If it can't run in an A/R, that takes away a lot of customers - and bolt guns can shoot anything, including all the popular .20 caliber "upgrades, like .20 PPC and larger that we already have. Seems the only reason the .204 Ruger exists is for those whom don't hand load, as I think most folks would rather use inexpensive and always-available .223 brass if they reload. One can buy Lapua .223 brass for not much more than most factory .204 Ruger. If a Vartarg or other smaller .20 caliber cartridge had any benefit over the .223-sized cases, I'd use it. But to have a smidgen less recoil, a few more rounds shot before barrel cooling and a few grains of powder saved are the only attributes over the .223-sized cases. To me, that just doesn't offset the velocity loss, increased brass cost and availability - so I just can't see a smaller sized cartridge coming out at all.
So the question is whether a manufacturer thinks in terms of unseating the .223-sized case with something that is better. And enough so to lure the legions of re-loaders to use their brass instead of cheap and plentiful .223 brass. So - I'm thinking it will be a cartridge just a bit bigger than the .223, maybe more like a Grendel-sized case to shoot a 55 bullet or similar. The problem is - lead-free bullets may be more prevalent in the future. That creates a problem with the small diameter of the .20 in terms of throating. To get a 55 grain leaded bullet synched with proper throating would probably require a 50-grain lead-free bullet to have similar throating, magazine length, internal powder capacity, etc. No such "heavy" weight lead-free bullet currently exists that I know of in .20 caliber- so perhaps another bullet would need to be devised. I think that would be ideal.
Me thinks Wildcatting will NEVER die. 
