Do you want this to be a big reno? meaning, will you replace things totally or do you want to work with what you have mostly? will you add a toilet to the bathroom? if you replace the vanity, you can put in a larger one curving around the corner with a medicine cabinet above. If you replace the shower with one flat on the ground, you can also put in clear doors for bigger visual space. how about re-painting as well

Personally, it's better to have a wall between the bathroom and the toilet so no one gets walked in on. Definitely a good idea to keep the bath in case of reselling, although are you planning on going anywhere? If not, get rid of the bath and get a bigger shower I think.

Have you found a bath small enough that will fit in the space?

I love three in ones because if one bulb explodes, you've usually got a fair few backups before you have to completely fix the bulb (but that's just the laziness in me coming out). In my opinion, I don't think you need a demisting mirror because that's what the fan is for in the three in one.