There are possible changes in the development of German capabilities pre-September 1939 which would not cost a great deal, would probably not induce any British response but would make a successful Sealion more probable.
For example, in WW1 Germany carried out landing operations against the Baltic Islands
http://www.gwpda.org/naval/albion1.htm. It is just possible that someone could have made a patriotic film about this in the 1930's and the idea of Army-Navy collaboration might be picked up by Blomberg because it fitted his agenda, which was aimed more at cooperation in procurement. Thus the pre-war development of something like the Marinefährprahm
http://www.german-navy.de/kriegsmarine/ships/landingcrafts/mfp/index.html or even simpler the Siebel Ferry
http://www.german-navy.de/kriegsmarine/ships/landingcrafts/siebel/index.html is not completely improbable. Given the strength of the Royal Navy, the British would probably accept that this was designed to give Germany a capacity to intervene in Estonia without passing through Lithuania.
Secondly, the British would not have reacted if German torpedo development had been more competent. OTL they had no idea that the German torpedoes were “wooden weapons”
http://www.uboataces.com/articles-wooden-torpedoes.shtml. The air launched torpedoes were also very poor. One possible POD for the development of better air launched torpedoes was the German work on the development of a closed cycle motor
http://www.ww2aircraft.net/forum/weapons-systems-tech/junkers-km8-torpedo-engine-7556.html (hope that nobody here has been banned from that site). Now while a technical achievement this was almost useless as it was powered by petrol (gasoline) and nobody wanted that inside a U-boat. Also development went through a hiatus as the original designer was Jewish and was sacked in 1934. So assume that the development is resumed quicker and somebody realises that it should be used for air dropped torpedoes as the Luftwaffe is happy to handle aviation fuel.
However, the critical issue is that Hitler has to decide in September 1939 that firstly he can only hope to win a short war and secondly that Britain might not make peace even if France were defeated. If he orders planning for Sealion in September 1939 and also orders that no resources are to be devoted to post 1940 developments if they can be used to make victory more likely in 1940, then a non-ASB successful Sealion might occur.
My thread
https://www.alternatehistory.com/discussion/showthread.php?t=167056 is not an attempt to predict the most probable outcome but simply an attempt to write the back story for the common German stories such as
http://en.wikipedia.org/wiki/SS-GB or
http://en.wikipedia.org/wiki/Collaborator_(novel) (see
http://tvtropes.org/pmwiki/pmwiki.php/Main/GodwinsLawOfTimeTravel).
There are many plausibility worries for that thread. For example, Iowa sailed on a war patrol 12 months after launch while Bismarck sailed 2 years and 3 months after launch. I assumed that very high priority might squeeze that down to 18 months but German Naval dockyards did not have a good reputation for hurrying. A thread elsewhere
http://www.kbismarck.org/forum/viewtopic.php?f=1&t=3890 mentioned “In the Indian Ocean Kruder captured intact the Norwegian tanker Storstad. He ordered that vessel to be converted into a minelayer to mine the ports of south-eastern Australia. The Pinguins' chief engineer said that he could do the job in two to three days,
but if it was left to a shipyard in Germany the job would take about three weeks. The job was done at sea in two days, in three the mines were all aboard.”