Before World War two, American vehicle designers had the luxury of following designs from Europe so they seldom came up with particularly wacky innovations.
After World War Two, when the uncertainties about the shape of future conflicts created a spirit of experimentation (large budgets may also have been a factor.) There were a series of unusual ideas and designs in all branches of the services, especially in the 1950s. Here are a few wacky ideas from the ground forces. Davy Crockett Nuclear devices launched by a Jeep. Ontos recoiless rifle air droppable tank destroyer The second didn't turn out to be actually useless in combat.
[The Air Force also had it's share, like the various ways the ultra long range B36 would carry it's own fighter support attached to the plane] (https://en.wikipedia.org/wiki/FICON_project) However the mid air refuling techniques that ultimately became successful aren't that much more impractical sounding, they just happened to work when people tried them.
I think one thing to keep in mind here is that there is nothing "insane" about getting bizarre concepts to the prototype stage. You never know what out of the box idea is going to have military value. The important thing is not to build large numbers of the more impractical prototypes after testing.
During the war, I'd say nothing close to the Ratte, of coarse there's the T28 Heavy tank/T95 GMC, and the T92 for the sheer size of its armament (240mm).
It gets weirder post war but for heavy tanks it really just culminates into the T58, which has an auto-loaded 155mm gun.
The beginning pages of R.P. Hunnicutt's "Abrams: A History of the American MBT" is what's full of unusual designs. There were proposals for quad tracked tanks (like the fictional Mammoth tank of C&C). The Chrysler TV-8 that has the crew armament (smoothbore 90mm T208) and power-plant all in its turret, also one of its proposed engines uses nuclear fuel. The R-32 was supposed to be a straight up nuclear powered tank while the R-35 would have had a 105mm recoiless gun, which could fire at 300 rpm.
