Just finished capping out my mansion 400/400, and in the end walled off several large sections of rooms just so things wouldn't feel so spread out and empty. I hate having to do that, but even with doing that, things still feel a little sparse in areas. Granted, it doesn't help that I have easily 100 flower vases around, 72 of which interactive items. Another 50 or 100 item slots would be wonderful, but honestly, just being able to stack some items would help so much.

Give us a wall mounted item that functions as a vase for several flowers/flower arrangements(possibly a garden thing so it's 1 item with 4 slots rather than 2-4 individual planters in the house). We've been given so many new and interesting flowers and other table top items that it's staggering to think that none of them are stack-able, and there is still no good solution for making things not flicker/vanish once you hit the soft cap.
Give us large partitions that cover the length of two. Give us pillar-ended partitions. Little things like that would help so much with saving on item slots. It might only free up 5-10 for some houses, but honestly, those 5-10 can really make a difference. So many item slots go into just walls/pillars/room dividers. Seeing ways for some of that to be reduced in the item counter would me amazing.