There are lots of tub and shower options available, but for true universal design, the best option is a walk-in shower with a flush, zero-step / no-curb entrance. Large rectangular walk-in (or roll-in) showers are typically 30×60″ or 36×60″ and should be open from the widest side. Non-rectangular showers can work well too, but make sure there’s at least 48×48″ of floor space for the greatest flexibility of use.
As for the idea of using a shower curtain or a shower door, it depends. Universally-designed showers should be barrier-free. Avoid using a door unless you’re sure it’ll be out of the way when opened or closed, and make sure any user can open/close it from inside while maintaining the ability to reach any mobility equipment that needs to stay outside.
Be very careful with integrated seats.
It’s typically best to avoid integrated seats and only add them as-needed. A strategically-placed built-in seat will provide options for many people, but there are lots of user variables that need to be considered, such as balance, effects of pressure on skin, and the way a user gets on or off a seat. If any health condition exists, it is important to involve a professional (specifically an occupational therapist) who understands the value of seat type and placement for safety and optimal function.