A slab floor and walls in Granite, marble or Quartz will give you a seamless grout free alternative that is easy clean. I have included a picture of work we have done for reference.

A slab can be great, but I'm not sure exactly what part of the tile you need to remove or what problem you're solving, so it might not be the right solution. Are you sure this isn't something that could be solved with regrouting?

It's very hard to remove only part of existing tile -- you tend to break or crack surrounding tiles, and even if not, it can be hard to get everything to make a smooth plane again. And that's with matching tile. Usually I'd say completely retile the shower, but with the continuous pattern, I can see that just about any approach is hard. The best I can think is a deliberate contrast, maybe a gray tone taken from the current accent tile, and use it to make an interesting pattern around the shower . Maybe you could get a floor tile that's a mix of that new tone with something similar to the current wall or floor tile. The best way to sell any of this as intentional would be to use a little of the new tile somewhere across the room, such as behind the vanity. But honestly, it's going to be very hard to get an effect that's anything other than a two-toned automotive paint job.

My inclination would be to keep the tub deck and bench and take out all of the wall tile all the way around, even if it continues past the tub (I think I see bullnose in your second pic, but not sure), and then replace the floor and walls with something that coordinates with the existing tile but gives a new look. You might even end up making it more updated. Anyway, I think that's the only way you're going to be really happy with the outcome. Sorry.