I would like to tile a double shower cubicle with 20cm x 20cm travertine tiles by tiling over the existing ceramic tiles (removing the need to destroy the plasterboard wall by removing the old tiles - these tiles seem to be attached really well). Is it OK to do this? Also, in a shower, will the grout and tiles become dirty over time as it is travertine and if it does how would I clean and reseal the grout if this happens? If this becomes all too difficult, I will trade the tiles back in and use easy ceramic ones!
No reason why you should not tile over the existing but do use a good quality waterproof adhesive and allow a week for the adhesive to dry before you grout. The problem with travertine is that it needs to be sealed so you will need to do this before and after grouting and then once the seal has gone. Cleaning tiles in a shower to remove limescale and soap residues is a regular task - I would recommend you exchange the tiles for something with a smooth fired surface.