Ahh, the conduit is a nice one. Every once in a while I snap back to attention after staring at mine for a while and contemplating my Minecraft existence. I think the slime block is my fave, though some people would recognize it by it's scientific name, "Iskallium"...
Anyways, I highly recommend using a few of the slightly transparent green gelatinous cubes to descend from great heights, particularly if you don't have many spare elytra. Harvesting the slime can be a pain (I use cheats) but outfitting your server with bounce pads will serve you well. If you don't miss, that is...
I have aims to create a minigame where the objective is to bounce safely from the build limit to bedrock, but I haven't quite figured out how I'll manage it yet. At the time being, I know where it's going but not exactly what it will look like. Waterfalls and slime go together like, well, waterfalls and slime. The honey block is certainly going to be included as well, still figuring out how it works. Glowstone is another favorite, (not the vanilla texture though. yuck) and of course there's always stone, which is certainly the block I use the most. I started using stone because I wanted my buildings to "blend in" with the stone around them and I despise cobblestone pretty much entirely because it doesn't occur naturally.
As far as mods go, Optifine can transform glass from being a real eyesore to virtually invisible (using "connected textures"), so I think that is the most useful adjustment to make. Optifine hasn't yet been released for 1.15 but the "zoom" feature is equally elegant, and sorely overdue in the vanilla game. I use it for about 90% of my screenshots and while it obviously isn't a block, It essentially is like walking around your map with a small DSLR (camera) that is just super handy, fun, and simple to use. The Replay mod is also super cool but I haven't been able to use it much yet.
Here's to dirt slabs, colored slimes, and smithing table functionality in 2020! I can't wait to....smith...some....tools? I honestly have no idea why there are any GUI blocks other than the crafting table sometimes...
