I agree, adding jet engines would make little sense. A much better bet might be to use jettisonable rockets, perhaps similar to the ones used to launch V-1 missiles - they would give enough boost for takeoff, and then simply drop off immediately after. However, as already said, the presence of a heavy bomber wouldn't really help Germany at all. Apart from anything else, it is entirely likely that Hitler would have meddled around, wanting the Luftwaffe to stop buying something useful like fighters, in favour of building modest numbers of these heavy bombers. It could prove to be the undoing of the Luftwaffe, perhaps shortening the war a bit.
Another bomber 'what-if' might have been the RAF having a British B-29 type bomber, with good high altitude performance and a good bombload. Using a common-section tubular fuselage, and semi-automated guns (.50cal, 15mm BESA or 20mm Hispano?), it could have helped with survival of the heavy bomber force. The alternative might have been for Britain to abandon heavy bombers as their primary bomber force, and switch to using the smaller, more agile, Mossies, which could at least fly in low and hit their targets (albeit with smaller bomb loads). Building hundreds of twin engined wooden bombers might have been cheaper, and yet potentially as effective, except for the area bombing. Obviously, if you really want to shorten the war a fair bit, you could have the RAF having more transports and gliders/glider-tugs. On Market Garden, a fraction of the intended force was actually landed in the first wave, hence they needed to waste time securing landing grounds for the follow up waves - have the entire Division landed in one go, and it might not have been a 'bridge too far'! This could see German forces falling apart, and the war being months shorter, and possibly a somewhat different post-war situation (Churchill potentially winning an early '45 election). This brings many potential what-ifs, e.g. no Russian Nene engine, so a poorly-engined Mig-15, stronger post-war forces etc... Heck, it could even go further - Army largely demobilised (no BAOR), relying instead on airpower and naval power!