Overlooked items that could potentially be forgotten and not added before they move on to TS5 (but I hope not):
Pool tables
Urinals (they added toilet stalls but forgot urinals for male restrooms)
Diving board (not the diving platform from Get Together, but a regular diving board like in TS2/TS3)
Pool slide
Pool bar
In-ground hot tubs
More types of gym equipment (stationary bikes, bench press, pull-up bar)
Playground slides for kids (we only have one for toddlers)
Playground merry-go-round
Seesaw
Trampoline
Hammocks
Separate cooktop that can be placed on top of counters
Separate wall oven (with double variant)
Blender/Food processor
Deep fryer/Air fryer
Juicer
Trash compactors
Landline phones (not really much used nowadays but it's cool for more vintage houses)
Drums
Regular arcade machine (we only have a giant one from Get Together, but no normal-sized ones)
Vending machines
Solar panels and water wells (useful for off the grid lots, they would provide electricity and water for those lots, or make bills cheaper in regular lots)
Regular-sized telescope
Soccer goal (although this is unlikely since we got soccer balls in DU, even though they forgot soccer goals)
Functional/animated elevators
Not really overlooked (we'll probably get them eventually), but we're still missing them:
Bunk beds/Loft beds
Cars and garage doors
Ladders
Spiral stairs
