I am thinking, but its not much to go on:
1) Instead of complete unrestricted submarine warfare in 1915. The Germans start USW in a few smaller areas. The North Sea and channel east of Ramsgate-Boulonge. The med east of Malta. Prize rules elsewhere. Less likely to annoy USA and really didn't have the numbers of submarines to do major USW anyway. Should be able to maintain this level. Gradually increase to all of med. All of channel. then full USW by late 1916. Get the USA used to the need for USW gradually.
2) Just build more submarines in 1916. Give up idea of war winning land offensive (i.e. Verdun) if that allows more submarines to be built.
3) Forgo building the larger types, nothing bigger than the UB III class, build more UB II, UB III instead.