Master bathrooms are often an extension of a home's master bedroom; they're typically located within or adjacent to the master bedroom. For this reason, some design decisions about your master bathroom may be simplified. For example, you may want to mirror or at least emulate the master bedroom's design.
Your first task when considering a master bathroom is to determine how, and by whom, it'll be used. Most commonly, two people will use a master bathroom. This means you'll need to design the bathroom both for co-use—which is to say, both people using it at the same time—and for co-storage, meaning the accessories, linens and other bathroom implements of both folks individuals will need adequate (and likely separate) storage space.
One approach that's popular in master bathrooms is a long countertop featuring a double sink. This will allow for both people to use the sinks at the same time, and it should also afford plenty of space for accessories to be placed on the countertop during grooming activities.
Many master bathrooms add elegance to efficiency, with features like a claw-foot or whirlpool tub used as a centerpiece of the bathroom. Other elegant but useful features can include high-end flooring options, bench seating with built-in storage for linens, or a separate, private toilet area.
In terms of design style, the master bathroom often follows the lead of the master bedroom, but this isn't a requirement—especially if the bathroom isn't located adjacent to or within the bedroom.