I would keep a tub in a master bath that already has a shower even when there is a tub elsewhere in the house because many buyers like/expect to have that in a full en-suite bath.

Go with a bigger shower. But that sounds like a HUGE shower. Maybe you need a small linen closet in that space?

huge showers rock. mine is 4' x 8' and i wouldnt change a thing. the 'far end' stays dry so you can have your towel and robe on a hook and never have to drip on the bathroom floor.

let the next owner of your home do what they want, its your space now.