put a relief tile w vines/flowers/ design/ etc....intentional difference is much nicer than "trying to blend" measure the inches...go to your local art association/ potters guild or co-op /etc....there will be people doing pottery that make tiles...just order 6 or 8 tiles ...someone probably has some ready right now..[delays in art fairs w covid].... ..adjust the grout dimension to work and it'll be good. better than the mosaic.

I think you should just remove the tile. Those mosaic tiles don’t look good at all.

It's not a large section of wall, and you're going to have to remove the half pieces anyway. I'd remove the entire section of original tile, and replace with something that complements the rest of the kitchen.

The glass tile you posted would not be a good look, it would look like the patch it is, and you would not be happy.

I would never put any "relief tile" with vines, flowers, etc. It does not work there, is more complicated in choosing it and installing it than you need to be, and the space is too tight in any case.

I'd remove all the tile that is behind the cooktop, and put a stainless steel panel there instead. A stainless steel panel is not expensive. It is a nice clean look. It can be purchased online in the size you specify, or most Home Depots have them and it can be cut there to the size you specify. There are instructions online how to install the panel as a DIY, and videos too, or you can hire a handyman to do it. The panels are easier to clean too, since there's no grout.

We do have the same travertine tile on the opposite wall so I'm not sure if that makes a difference

If you choose the stainless steel panel that I discussed above, it doesn't matter what the rest of the tile is on other walls. The stainless steel panel is a neutral and goes with everything, and you are just placing it behind the cooktop.

Salvage the tiles from each side and put them behind the range. Patch and paint the drywall on each side.

how do you "salvage tile" from the sides? I tried that once and stopped because of the destructive nature . Is there a video of someone actually doing this ?

It will mess up the drywall and may require new pieces of drywall on the sides. But I would not want to spend a lot on anything new if the kitchen will be renovated soon. It will only be worth it if the homeowners do it themselves, taking that apart carefully would probably cost too much if a carpenter did it.

if those tiles dont get chipped and break then drywall repair is not a big deal. I guess she can have a go at it but the risk is the tiles get damaged ......that is my impression w this type of thing. one of those projects where you dont know until you get in there. Person w courage and time..... just do it.....this is the cheapest and it looks pretty nice. 5 dollar sample pot of chosen color when painting the drywall is all you need

Stainless steel.

Thank you everyone for your suggestions and input! We definitely want to go with the easiest and affordable option since a remodel is in our future. We do want to tackle this project ourselves so you have definitely given us some things to think

As @herbflavor mentioned, there is a risk in extracting your tile from the sides to move it to the center, which is that the tile could get damaged. Travertine is a type of stone that is easily chipped.

Furthermore, from the OP photo, the tile to fill in that space will need to have a half row of the travertine tile at the top. In other words, one row of the transplanted tile will also have to be cut in half. Do you have the equipment and skill to do that? Or, do you want to pay a tiler to do that? I stand by my original recommendation for removal of the tile behind the cooktop, and placement of a simple stainless steel panel per the pics I posted.