I have a solution. Let's ban all cartridges in odd numbered calibers 30 caliber and under. We'd have 20, 22, 24, 26, 28, and 30 calibers to work with. Use any case you like, but it has to be an even numbered caliber. To keep things fair everything larger than 30 caliber has to be an odd number. We could have 31, 33, 35, 37, 39, 41, 43, and 45 caliber cartridges.
Before I start getting hate mail, in case you haven't figured it out that was meant to be tongue-in-cheek and not serious.
But if you think about it that's basically where we already are. 30 caliber and below almost all of the popular cartridges are in even numbers. There are a few 25 and 27 caliber cartridges, but they've never seen anywhere near the popularity of the even numbered cartridges that bracket them. And you'd be splitting some pretty fine hairs to argue they offer any real advantage over comparable 24, 26, or 28 caliber cartridges. It's just a lot easier to find loaded ammo and reloading components for the 24's, 26's and 28's that do the same thing as the 25's and 27's.
Above 30 caliber almost all of the popular cartridges already end in odd numbers. We have 33's, 35, 37's, 41's and 45's. For whatever reason none of the even numbered cartridges have ever really been popular. At least in this country.
None of this was by design, it just worked out that way.