A few furnishing ideas;

Double Partitions - Exactly what it says on the box, a version of existing partitions that's just twice as long. Could even go further with a triple version, would save a few on the furniture limit.

Windowed Partitions - Essentially something similar to the Open Partition, but for a variety of designs. On that note;

Add the missing Partitions - Rename the Mahogany Partition Door to Riviera Partition Door, add a Hingan Partition Door, Hingan Half Partition, and so on. Blank X Partition Doors, Blank Half Partitions, Blank Open X Partition, the list goes on. I'm almost certainly missing some things here, but you get the idea.

Half Pillars - Pillars that don't go all the way to the roof and have a nice "rounded" top. Mostly thinking of the entrance way in Mansions for this, where there is no roof and thus Pillars look a bit off IMO, but Half Pillars would also just work nicely with Half Partitions.

Sin Eater Sculptures - To match the Vodoriga and Gargoyle Sculptures perhaps... In general I'd just like more Sculptures like those two, just of nicer looking things... Doesn't even have to be Sin Eater based, I just though that would pose a nice mirror to the existing ones, and honestly a Forgiven Venery Sculpture would be great... Possible drops (crafting material or otherwise) from Mt Gulg, or even Lost City (Hard), given there were already statues of Sin Eaters there...

Sports based furniture - This is something others can perhaps expand on, but I'm thinking things like a Kendo Set (Rack of Bamboo Swords, some of the armor strewn about perhaps), a Boxing Ring, some of the Archery furniture kicking around the Archers Guild, that sort of stuff. Leading off that;

Weapon Racks - There are all sorts of these already kicking around the game. Bows nicely stacked away, Swords, Spears, etc. I would very much like these as furniture options.

Indoor Fountains - Currently we only have one, Llymlaen's Embrace. This is fine for some things, but anything involving symmetry and/or tight spaces? You can only have one per estate and it is frankly huge. Just some small fountains to place by the front door, that's all I want.

Entertainer based permits - So we have permits for NPCs that have useful functions; Shops, Repairs, Melds, etc. I'd like something similar to this, just for having NPCs that do "pointless" things. Like when you enter the Gold Saucer, you've got those NPCs juggling, let us hire them. Let us hire a selection of Bards playing various instruments. Best way to add Mime to the game? Plenty of ideas you can have for this, but one important detail; Make them Tabletop furniture. That way we can place them on things like the Troupe Stage... Actually, as a side point, I'd say that goes for the existing Permits and Mannequin... They really should be Tabletop items...

A Toilet - I feel this is just the natural progression after adding the Washbasin...

Oh, and a couple features I'd like to see;

A Seasonal or Mogstation tab for the Preview Furniture option. I don't like the Mogstation, but I feel people should be able to preview furniture before they go ahead and dump real money on them... I mostly just want this though so I can preview seasonal furniture that I have lost somewhere on a Retainer...

A Preview Fixtures option. Simple enough, same logic behind the ability to preview furniture, let us preview how Lights/Walls/Floors/etc will look.