Right off the top of my head I can say Patch. He was in command of the “American Division” that saw action relieving Marines on Guadalcanal. I’d have to think for a bit on who the others would be. Did you know that Patch was the first of the main Army Generals in the European campaign to die after the war. Patch had chronic lung problems that predated the war. It finally got the better of him in November of 45.
As far as who would have taken Ike’s place in London. I don’t see anyone wrenching MacArthur from the Pacific theater, but Devers would be a possibility. I think Bradley would be more likely since he was rated rather high in Marshall’s “black book”, though I think Simpson would have been the best suited if not Ike. Like Ike, Simpson was very good at listening as much as ordering which made him a quite, but effective General for the 9th Army. Let me throw this one out for a sleeper pick, General McNair, he was pretty highly regarded by Marshall and I think he was a great logistical general.
You are correct with Patch. The other was “Lighting Joe” Collins who led a division in the Solomons after Guadalcanal and then led VII Corps in Europe. I’m still leaning toward Devers as the London pick; he was high in Marshall’s book, and Ike knew it. He and Ike were rivals. That’s why his army group was the “red-headed stepchild” of the European Theater when Ike was Commander of SHAEF. Devers forces did very well, better than Patton, fighting through rough terrain and terrible weather in fall 1944. All on a shoestring budget.
Marshall had an eye for talent and putting the right man in the right job. That’s why I think he would have been reluctant to part with McNair. He needed someone at home to build his army, and that guy had to be highly competent. A pity McNair doesn’t get more credit, other than having the distinction of being the highest ranking army officer killed in combat when the bombing preparation for Operation Cobra landed short.