Restore all ceilings, floors and doors, but keep the walls somewhat 'neutral'. People love original features, but most of them are overwhelmed with the amount of patterns. Use solid colors (not necessarily white/beige) to make the features and furniture stand out more. Attention grabbing. Find an expert electrician (with experience in old buildings!!) as there's plenty of wiring to be worked into those walls. And an engineer that can help you with the best location for bathrooms. Look into bathroom modules/pods, as you don't want to ruin those lovely ceilings with too many partitions (they can be 210cm tall for example). Plus a really well thought through lighting plan. Comfy beds and (white/champagne) bedding. Consider keeping the architectural features art-nouveau (soft and swirley), while using more art-deco furniture (straight, glam, functional, geometric), because it'll create a more modern feel. Look up many images, as both styles are still quite popular and/or well documented. Examples: https://inspirationdesignbooks.com/blog/design-trends/design-trends-art-deco/ https://www.elledecor.com/design-decorate/trends/g26798427/art-deco-bedroom-ideas/ https://www.danetti.com/blog/art-deco-style-modern-interiors/ https://artnouveau.club/art-nouveau-vs-art-deco/ http://theinterioreditor.com/interior-inspiration/interior-styles/the-new-art-deco-art-nouveau-modern-style/ And visit Prague. Or Berlin: https://www.hotelartnouveau.de/en/gallery/ 

Those fireplaces are some of the nicest examples I've ever seen! I think it would be such a shame to take them out. In the 50s people were scrapping old Victorian cast iron fireplaces because they looked dated and look how tastes have changed. Art Deco is just starting to become the new thing again. If you really hate them, you will sell those without a problem.. I fitted an old stove into mine and I'm happy with the result.

The problem is the stark cold white walls against the soft warm browns of the fireplace. I don't recommend going all beige but try colours from the same warm spectrum as the fireplace and I think you'll find it looks more natural in the space. Also, the round candles don't work with the squares and oblongs of the fireplace; find other accessories that enhance rather than scream at the shapes and colours and that could make a huge difference. So could a mirror or picture above it - it would shift the focus so the fireplace doesn't dominate. Work with it, not against, then judge it. These fireplaces are handsome; trying to pretty them doesn't work.

A few years ago this would have been my dream bathroom as I was obsessed with Art Deco! If you are going to keep it in its entirety, then I would go with a plain white floor and defo lose the curtains and pelmet -- window film or half shutters along the bottom half perhaps? I know the floor tiles are of the period, but it is pattern overload. It is bloody amazing that the whole bathroom looks in such great condition though. What a find! Would love to see what you end up doing with it.