Obviously, it's all about what Brandon wants. He turned down a 5 m player option for a reason- he had a career year, and when you have a career year that coincides with a contract ending, you cash in on that. 3yr/24m or 4yr/30m is not out of the question for him.
As far as the celtics are concerned, Bass is a perfect fit for THIS incarnation of the team, but I'm not sold on his value to us in the post-Big-3 era. By that point I envision Jeff Green and Sullinger will be eating up a whole lot of minutes at the 4 spot. I really don't think that we should sign Bass for more than 2 years when his role might be diminished by the 3rd year. We should aim to retain his services in the present but not at the expense of our future financial flexibility.
I think that we can safely offer Bass something in the area of 2yr/15m. Maybe a little bit more money than that, but I really don't think we should add years.