28=135 and the 70-300IS serve that function quite well. Both are pretty decent lenses especially on a body capable of MFA.
The 28-135mm has a large zoom range, so reducing it from a 5x zoom to a 3x zoom should improve image quality. It also has an older implementation of IS, which can be updated. The 70-300mm can also benefit from newer IS and, perhaps, ring USM.
This.
Or get a 5D1 with EE-S screen and some vintage lenses or the 28-135mm and 70-300mm mentioned above (or the Tamron 70-300mm).
I just don't see that coming to fruition. If you want expendable, the body isn't going to be unless you want to use them with an old film camera, ie small market, or an older 5Dc. Otherwise just get a newer model crop camera and use the 18-55-250 combo.
Maybe expendable was the wrong word. Let's call it portable full frame. Much like the 40mm f/2.8 is very small and portable, so too can a 28-88mm and 88-400mm be. The 6D is a cheaper, lighter weight full frame camera that these can be paired with.
But I guess it's not in the cards because these lenses would be in no man's land. There are already the crop bodies with these lenses, as you mentioned, which are meant to be an introduction to DSLRs. An upgrade to full frame usually means upgrading the lenses to constant aperture zooms and/or primes as well.

