then you'd no longer be able to support it with Soyuz, Progress, or Proton launches out of Tyuratam. The ISS is already in the lowest orbit you can achieve with a direct ascent from there. There are plans to launch Soyuz rockets out of the same facility where Arienes are launched. When that capability is available, then a lower inclination would be supportable. The best orbit for assembling spacecraft for other missions such as the moon or Mars would be about 28 degrees. Anything lower than that and you couldn't reach the ISS from the Cape.

As for putting ion engines on the ISS, it's a good idea but I don't know if the ISS has enough electrical capacity. That new engine requires over 12 kilowatts and up to 25 kw for full power.