In 1905 Mauser developed a semi-automatic rifle that was plagued with trouble being able to handle the miltary cartidges of the time. Supposing he corrected this problem, perhaps by developing an intermediate cartidge or by having a way to fix the problem with the cartidges of the time, how does this affect World War I and what are the effects in general?
The effects on WW1 are probably pretty marginal.
It's not impossible to
design a semiautomatic rifle for 8mm Mauser; France had
a semiautomatic rifle in the works at the same time, and the ballistics are in the same ballpark. (Even though it was a huge step up from the Lebel, it was doomed because there wasn't ammo stockpiled for it. An intermediate cartridge would have the same problem and be a notable step
down in killing power; good luck selling that.) The main challenge is producing enough of them; most semiautomatic rifles from that period were built to exacting standards and priced accordingly, which is why Mexico abandoned the Mondragon (they couldn't produce it domestically.) It's not insurmountable, but it
is an added expense, and it's markedly less effective than smokeless artillery and gas.
(As an aside, France also bought semi- and fully-automatic Winchesters for pilots, and later for trench raiders. They weren't far removed from M1 carbines, but
those becoming widespread in the Great War would have a big impact by WW2.)