Bump... hoping for some feedback... thanks in advance!

Honestly---I'm not quite sure what you want. You have a window that doesnt move around because it is stationery but you want advice where to place it. You want to add a shampoo niche.

Do you want to tile inside the walls of the niche?

I think your window is so pretty and would not want to add a niche on that wall to detract from it. Plus, you don't want a niche on an outside wall.
Your oval window is not at all modern but a 12 x 24 tile is very contemporary. I would rethink the shower wall tile. How in the world are you going to find a tiler who can cut curves to go around that window? Sorry, I just saw you mean to try to use a mosaic in the window area. If you use the larger tile, I'd take it up to somewhere below the window and do the mosaic from there all the way to the ceiling. I might do a smaller wall tile, tho.
If you do a niche, I'd do it either on the fixture wall or the opposite wall. I just installed a shampoo shelf that I think would look nice with your curved window; it's also a grab bar. Maybe instead of a niche?

https://www.amazon.com/Invisia-Shampoo-Integrated-Brushed-Stainless/dp/B077YR8P36?th=1
Several other companies make something similar but they look more like utilitarian grab bars to me...this one is a little more refined.
I just googled round windows in shower and found this lovely example. And it looks like they've used a 12x24 tile, too.
But, honestly, based on what we see on these forums, it's difficult enough to find a tiler who can install a straight edged tile, let alone cut a curve.

IMO do a glass shower screen forget the shampoo niche. I have no idea why people want to high light shampoo bottles ever . I think that tile is quite busy for a small space so keep everything else very plain. Use a nice shower caddy for shampoo hung over the glass door I like a hinged shower screen to allow a bit more coverage and also lets you reach the faucet . I love the window and no window covering is needed but since it will now be in the shower it needs to be able to withstand moisture so check that out. In our house we have international students tha share a bathroom and they just have a basket with their bathroom stuff in it that goes back in their room when they are done I aslo provide guests with a basket for all their bathroom stuff in the guest bath I see no point in having all that stuff on display.

I agree that small tiles (either mosaics or penny rounds) would be good to line the window sill around the window.

I disagree with Patricia that a hanging shower caddy is preferable to a niche (really???). Niches are typically at the foot of the shower/tub so that the water doesn't spray it.

You could use the small tiles to line the back of the niche, to tie it in with the window.

Thanks for the feedback. I like a niche as well - have one in our main bathroom. @weedyacres, when you say the foot of the shower, do you mean the farthest back wall, away from the faucet ? Just clarifying. Thanks!

Here's kind of the look it sounds like you're describing. Can you tile the side of the tub too? Notice that there's an arch on the left side of this picture, and they just kept the same pattern for the tile. I think your window will stand out but be clean and crisp without the mosaic. Can you put your niche at the foot of the tub so it's not a highlight as you walk into the room? And I'd keep the same tile in the niche as that is the more up-to-date look now. Here's a good article about them.

@stiley-
That is very close to our main bathroom (not the arch, just the tiles & design with the tub). We aren’t replacing the tub so unfortunately, can’t tile alone the side of it. Can you clarify where you mean by the foot of the tub ? Assuming you mean NOT where the faucets/pipes are ?

Regarding the window, what we have now uses that sort of design and the tiles don’t lay down well (flat) along the arch and thought finding a mosaic would lay flatter. Also, we are now tiling all the way to the ceiling so do we need to tile all around the window ? Really appreciate your feedback.

On another topic, what size tiles are inside of that tub?

I just found that images on the interwebs but it looks like those tiles are 12x24. For the niche, I'm talking about the short end of the tub opposite where the controls are. That's just so it's not staring at you when you walk in the bathroom door. I referenced the arch because it's curved like your window, not that you should add an arch. Just pointing out that it's not highlighted by different tile. I think you should do one kind of tile throughout the shower and up to the ceiling. Not the large format + a mosaic around your window and niche.

The other thing you might consider to keep your space pretty after construction is decanting your shampoo into pretty bottles like these. They look like amber or green glass but are plastic. I do this and labeled them with my label maker. Especially since this is a guest bath and you won't be refilling frequently. Although they hold a lot, so even with mine in our primary bath, I don't have to refill very often.

@Rho Dodendron-
I have several things I’m solving for:

1. The stationary window that has an oval shape and the best way to tile the flat surface surrounding the window (currently it is tiled 1/2 up and exposed along the non tiled portion of the wall — our new tiles will go to the ceiling). How far to tile (all the way around?), type and size of tiles
2. Niche placement - I like a niche but there is already a large window on that wall. Do I place it on the back wall, opposite the faucet ? On the same wall with the window? I want it accessible to the bathtub as well.

Hope this clarifies! Thanks in advance for all the feedback! Super helpful!

Kohler makes the Choreograph Collection that has a series of wall bars that can be used as grab bars or are wide enough to hold products on the ledge.

https://www.us.kohler.com/us/Choreograph-Shower-Wall-and-Accessory-Collection/article/900001.htm?subSecId=CNT123900023

What about a bench along the short wall and you can put your shampoos there as well? The window is a striking feature (it takes up a lot of the visual space of that wall) so do a careful tile installation and leave that wall alone. It’s what everyone sees when they walk in. A bench to the side doesn’t highlight all the bottles we collect in our showers. Personally, my hair product is in a pump bottle so I prefer a solid bench for my hair machinations.

@RedRyder -

Interesting suggestion! But a bench in a bathtub area? There isn't enough space on the back side of the tub so are you talking about building something up higher? Are you talking about at the left side of the tub (not on the faucet side)? A link to your suggestion would be awesome!

It won’t work with a tub, just a shower. If you’re keeping the tub then a stone shelf on that back wall would be where I put my shampoos. I’m just not liking anything in the wall with that unique window.n