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If you want a challenge, figure out how to build an iron farm, “starving villager” breeder, villager trading hall and villager-powered crop farms... all positioned so that they don’t interfere with each other but their chunks are close enough that they stay always loaded.
You could try building a base where the walls are literal aquariums full of fish. Obviously, for now that would just mean guardians or squid (but not both in the same tank), but with 1.14 you could seriously decorate the waterscape to your liking). That could prove to be a unique spin on a guardian farm, either as the actual functional farm itself or as a visible indication of what that site is for (with the actual drops-producing farm being kept far enough away to maintain peak operation while you're there.)
Another really cool build, but also a highly impractical one (don't do this on SMP, it's likely to be quite laggy), is to build a storage room that doesn't rely on chests to hold items (at least not as far as the visible portion of the area goes). Items would be kept in a dispenser/dropper chain leading to a point above your head, and would be dropped down onto you to automatically pick up. What doesn't get picked up gets sucked up by a hopper underfoot, recycling those items back into the shower loop. To get items, you'd just walk into the correct shower and start collecting automatically. Items could rain on a continuous basis, or they could be triggered by flipping a lever or some other redstone triggering device.
In 1.13, build a gigantic map room that utilizes the new item frame placement functionality (you can place them on floors and ceilings as well as walls). Use a three-dimensional slime machine to get closer to various parts of the map.
--An amazingly cool (and functionally useless) variation of this might be a gigantic globe-on-a-stand that you can parkour up to get a better look at various sections of the map.
--Another amazingly cool (and equally as useless) variation might be to do a space/void-themed room with various "planets" floating in the void having the maps of various bases and surrounding environs plastered onto them. The planets might have to maintain a framed map look rather than completely hiding the planet's block material across the curved surfaces of the planet sphere.