I personally think the most likely outcome is a hybrid calendar which combines the Martian sol, a 7-sol week, and the Earth year of approximately 355 sols. They might declare the the year to begin at the midnight Mars Coordinated Time which is closest to midnight UTC on 1 January on Earth.

I don't know what they'd do about months. I reckon the easiest thing to do them would be just not to use them, and use a week count instead. A lot of businesses on earth prefer to use week counts instead of months for planning anyway, so we already know that approach works. For medium-term planning, you can divide the sol-weeks of the Earth year into four quarters. (The Earth-year contains 50-51 sol-weeks, so that divides into quarters of 12-13 sol-weeks–some quarters would be 7 sols longer than others.)