There is a 6mm x 45 can be an improvement over 5.56mm NATO, but not the 6mm x45 Wildcat you are talking about seen today.
This 6mm x 45, the 6mm SAW:
The 6mm x 45 round designed by Frankford back in the early 1970s to ultimately replace both the 5.56mm and the 7.62mm. Unfortunately, it fell afoul of NATO standardization.
(Oh and wikipedia not withstanding, the aluminum cases did not have a propensity to "catch fire", they suffered from "burn-through", which has nothing to do with catching fire. It is when a crack develops in the head, and the hot, high pressure gas quickly erodes the aluminum crack to a much larger size, allowing dangerous amounts of gas to escape. The problem is in order for the aluminum to strong enough, it has to be hard, but when harden, it becomes brittle. And, even hard aluminum is easily cut by high pressure gas.
Curiously, this isn't a problem with larger (20 mm and larger) cartridges. But then larger cartridges can have proportionally thicker head webs, even though they can operated at higher chamber pressures.)
(And while they don't talk about it, I'll bet this cartridge really pissed off the British. 6mm SAW was about the same, performance-wise as the British .280 "Optimum" of the early 1950s, a step down from 7mm "Second Optimum" and a lot less powerful that 7mm "Compromise", all of which were nixed by the US as "not powerful enough"....)