Let's say that starting before the Nazi take-over, maybe 1930 or 1931, key figures in the German navy become convinced that carriers rather than battleships are the future of naval warfare. They do studies of contemporary designs and do paper designs of their own, then convince Hitler that fast carriers would give Germany a way to leapfrog the navies of their potential enemies. As a result, instead of building battlecruisers and then battleships, the Germans build carriers.
Historically, the Germans had one partially completed carrier by the time the war began and two battlecruisers. Figure that in this scenario they have maybe three carriers at the beginning of the war and maybe two or three more (carriers being somewhat smaller than Bismark-class battleships) by mid-1941. It's possible that carriers could be quicker or slower to build than Bismark-class battleships, so it's remotely possible the Germans could have as many as five or six carriers by mid-1940, and somewhat more possible they wouldn't have the two or three they built instead of Bismarck and Tirpitz until 1942 or even 1943.
Problems: (1)The Luftwaffe historically was reluctant to allow the navy to solicit carrier-specific aircraft from German industry, instead offering navalized version of land-based planes. It would take getting Hitler on board and keeping him there to get carrier-specific planes. That would absorb aircraft-design and production capacity, having some affect on land operations. (2) It takes a while to get a carrier tradition going. I'm not sure six years is enough. (3) A different German fleet would presumably mean a somewhat different British fleet at start of war. (4) The Germans would have to develop the ancillary equipment to make the carriers worthwhile, including effective air-dropped torpedoes.
So how would this play out? How might the Germans use their carriers? A German attempt to Pearl Harbor the British fleet at Scapa Flow early on? Carrier-based commerce-raiding in conjunction with the U-boats? Carrier raids on the US east or Gulf Coast in late 1941/42?
Would this hurt or help the Germans? The Atlantic isn't as good as carrier country as the Pacific--nowhere near as much maneuvering room and much nastier weather in the relevant areas. Ultra would probably play a role in limiting the impact of the carriers too.