I've heard people say don't cover tree roots. But in this case you would just be bringing the soil on the low side up to level. I won't be concerned if it were my tree. Maybe someone else has a different experience or opinion.

I think you can get away with raising only one side. The roots being buried so deep will be less effective due to a loss of oxygen at the deeper root level. The tree though I think will compensate OK by itself to the difference. Al

One time we built up an area with 2 x 4's, filled that with sand to make it all even, and then put flagstone over that, filling in the cracks by sweeping more sand it. It was a beautiful patio. I'm thinking if you did that, the roots would still get air through the sand. It wouldn't be like concrete. Or, you could do a nice wooden deck too!

A wood deck never even crossed my mind, that's a great idea! I wouldn't even have to fill at all, I could just put down some weedblock with gravel to hold it in place and put a deck over it. Thanks!