I've been rereading other's posts and found that I can still make a contribution: add the possibility to see how much of that item/furniture we already have when buying. I love secret bases and to decorate them and so I change the "theme" of it from time to time and it's absolutely terrible that I don't know how much do I already have in storage and always have to keep flying back and forth between my base and the selling points in order to see if what I have fits the space, if it fits the theme and looks good, if I need to buy more and how much.
Btw, we should be able to make a "sketch" of the base and then order all the furniture in order to avoid making so many trips and ending up losing money with mistakes (or allow us to sell the furniture back!).
Like a couple of people have said, add couchs, lamps (Shauntal's and Grimsley's E4 room were amazing), the possibility to rotate stuff, no decorations limit (or far expanded), the possibility to create space (but not by capturing flags nor relying on anyone else or being connected!), choose colors, two (or more) levels, dolls on the floor, etc..
I'd like the ability to create ramps when making/shaping a base, both going up and going down like a burrow and it could be an option to stairs when creating or using bases with more than one floor!
Like Lanturnman said (and perhaps someone else), they'll probably keep the square grid but in order to give a more burrow idea, I'd like to see some circular shaped bases!
I'd also like to have door in my base with the option to chose one from a list or just have one that fits the look of the base (for example, a bush door when in a base made of leaves in order to camouflage) since I never liked to leave my base open, despite knowing that no one would enter it! -_-
Btw, add the possibility to rearrange the furniture list in the pc, both between groups and between objects from the same group!
Last but not least, underwater secret bases with an entrance method the same as Seafloor Cavern and the walls being made of white (if in Route 126) or gray stone with moss covering some parts of it as sand as the ground.