What part of the pit is set near ground level--floor or top? Wood burning, or gas?
I am in a similar climate zone as you are (Louisville, KY), and open pits (of any kind) can become sumps and/or get pretty damp.
If the walls/floor of the pit are premanufactured in one piece, they can be pretty watertight and the entire thing could float if the ground water is too much--you would need to put drainage around it (clean gravel and soil separation fabric) and pipe out to daylight. If the pit is hand built and a bit porous, you can put the drain in the pit bottom and pipe out. We have some pretty heavy clay soils and high water tables around here, so I tend to go with making the pit walls/floor as porous as possible so that the hydraulic pressure is never an issue. I never use "dry wells"--I have seen too many silt up and fail over time.
I assume by ventilation you mean for the fire? I am not aware of any issues in an open pit. Usually the problem with gas fixtures is too much wind.
My main concern is that a concrete patio with a 4-6" concrete fire ring sounds institutional, cold and unattractive. Are you giving the concrete any kind of finish to make it unique? I.E. Polish/grind the fire-ring and sand-blast the field to create a different texture while exposing the aggregate in both elements?
Also, with gas, you will need stone, glass, etc. to diffuse the flame, otherwise it will be like standing around your stove.
You can avoid a lot of the design issues you're facing if you make the pit 18-36" above ground. Drainage can happen through weep holes at the base, at grade. You won't risk people tripping on a 4" curb and getting injured by the pit. You will be able to feel the flames better if it's higher and the structure can double as seating when the fire is off or you can make a seat/wall cap and sit on it when the fire is on...
yeah, yeah, yeah....I was waiting for the trip hazard comment :)
The concrete will be stained, curb/ring around firepit masonry to match house, etc.- the finish details have been discussed, more interested in ideas to help make it work effectively at this point, not look pretty (but thanks for the recommendations).