From what I recall some company down in Florida a few years back was making Mini-14's in .308. It involved rewelding the receiver & adding metal, but they were doing it. No ideas about quality, accuracy or safety though.
The .308 Winchester & the .243 Winchester require an action that's the same length. After all, the .243 Winchester is just the .308 Winchester cases necked down to .243. The .243 Winchester has a max COL of 2.650".
The .308 Winchester has a max COL of 2.750". That .10" difference isn't enough to let you use a different sized action. So a Mini-14 sized action wouldn't work with a .243 Winchester.
But I agree that Ruger should have came out with a gas operated semi in those calibers. Hell, if he had came out with a mag fed '06 based on the Mini design he'd have probably sold more of them than the Mini-14's in a given year (as long as they were priced about the same).
But if anyone wants something close to a Mini in .308 Winchester or .243 Winchester, rebarreling a Garand or M1A is feasible. & if ya want to trim weight & length then getting a "Tanker" conversion done to your Garand or a "Bush" conversion done to your M1A. Both will leave you with around an 18" barrel & a little less weight than a standard Garand or M1A.