The Germans already had the Marshall Islands prior to purchasing the remainder of the Spanish East Indies, so the German interest in the area was preexisting. Without Saipan, they perhaps get a better deal though.
I think Japan would still be interested in grabbing up Palau, the Caroline Islands, and the Marshall Islands. If Japan isn't spending time taking Saipan, they may beat the Australians to Nauru. That's a lot of phosphate money in Japan's hands.
Germany OTL put no efforts into developing the place, so Saipan would perhaps see economic development a couple of decades sooner than historically. Japan took an interest in developing the place when it captured the Marianas, and the US seems to have been of a 'make this place lucrative' attitude.
Many of the Japanese immigrants to Saipan of OTL probably still end up going. There was significant Japanese migration to American Hawaii and Philippines after all. The difference is that those people might not get evicted like OTL. The population of the Northern Mariana Islands is only a quarter Chamorro today, and over half east and southeast asian (Chinese, Filipino, Korean, etc.) so it would just be amplification of an OTL trend of sizable asian immigration.
