Originally Posted by
BIG_BIKE
...We are planning to build a custom steel frame (step through style)...
That just eliminated about 90% of your target market, I think.
But here's a frame idea that's not so common:
http://www.biria.com/bicycle/easy-boarding/cruiser
I don't know that the sloped seatpost would be a good idea in terms of handling a lot of weight. But that comes back to the issue of defining your target market. People that weigh 220 lbs don't need a custom frame, people that weigh 500+ lbs are probably very seldom on 2-wheel bikes.