The floor needs to slope 1/4" per foot to the drain from the furthest wall, so if the drain isn't centered, you need the slope dictated by the longest run for all of the other slopes making them potentially all different to achieve a level line all the way around the shower.
You need a hump of sorts at least so a shower door won't scrape on the floor, but a wheelchair could go in (although not an issue - yours is too small for a wheelchair except for maybe a small child size). Waterproofing it properly is a problem, too. I'd suggest you discuss this over at www.johnbridge.com. They help people build these fairly regularly. I suggest you look on www.schluter.com and read up on their Kerdi shower system.