You prompted me to go through and check what I was subscribed to on Reddit, so here goes:

Rubik's cubes

• r/anarchycubing
• r/CubeModding
• r/cubepics
• r/Cubers
• r/Cubinggore
• r/NewCubes
• r/twistypuzzles

!cubers@lemmy.world exists, but it is pretty small, and there isn't really a meme culture associated with it. Rubik's cubes are very visual hobby, so perhaps this is being hampered by image hosting costs/restrictions? Personally, since I can't post photos straight from my camera, so I haven't bothered to post any images.

Spaceflight

• r/AstraSpace
• r/DreamChaser
• r/fireflyspace
• r/JAXA
• r/NeutronDevelopment
• r/RelativitySpace
• r/RocketLab
• r/SierraNevadaCorp
• r/spacex
• r/SpaceXLounge
• r/StarshipDevelopment
• r/StokeSpace
• r/ula

For now, I think these topics can be adequately served by !spaceflight@sh.itjust.works, !spacex@sh.itjust.works, and !rocketlab@lemmy.nz.

• r/rocketlabmasterrace
• r/ShittySpaceXIdeas
• r/SpaceXMasterrace
• r/SueOrigin
• r/TEATEBMasterrace

We do have !spaceflightmemes@sh.itjust.works, but I'm keeping it on "life support" with Reddit content. I wish the Lemmy spaceflight userbase was large enough to support its own meme culture.

CGP Grey, Brady Haran, and Hello Internet podcast

• r/Bestagons
• r/bradyharanexplosions
• r/Flaggyflag
• r/HelloInternet
• r/HItrivia
• r/nailandgear
• r/PodcastPostcards
• r/T_HIP

I've made !hellointernet@feddit.uk, but things have been pretty slow. It doesn't help that the podcast hasn't uploaded in five years XD

@grant@toast.ooo has made a nice home for !cgpgrey@toast.ooo. (Psst, it's time to swap out the pinned Cortex thread for the latest episode :)

Tesla

• r/TeslaAutonomy
• r/teslacanada
• r/teslamotors

I think these are adequately served by !teslamotors@lemmy.zip and !avs@futurology.today. (Thanks @BrikoX@lemmy.zip and @sabreW4K3@lazysoci.al)

Lesser Musk Empire

• r/BoringCompany
• r/Neuralink

I've made !boringcompany@sh.itjust.works, but haven't had the time to grow it. For Neuralink, might a more general community for brain computer interfaces be viable on Lemmy? I'm thinking something like !bci@futurology.today.

Board games

• r/Carcassonne

We had !carcassonne@feddit.de, but no replacement was created when feddit.de went down. Any instance recommendations?

Futuristic tech

• r/deextinction
• r/fusion
• r/ITER
• r/wheresthebeef

!fusion@lemmy.ml and !fusionenergy@lemmy.ml exist, but they are pretty dead and I'd rather move off of ml where possible. I couldn't find anything for deextinction or cultured meat.

@Lugh@futurology.today, any interest in growing communities for these on futurology.today?

Harry Potter

HarryPotteronHBO

Closest thing is probably !leaky_cauldron@diagonlemmy.social, but it is also being kept on life support with Reddit content.

Miscellaneous

• r/JellyBelly

Nothing whatsoever for this.

• r/JoeyForReddit

Somehow, my favourite 3rd-party app still works on my phone. I don't know how, and I'm not going to ask questions. I am sad that u/codesForLiving hasn't made a Lemmy app. Anyone know what they're doing now or how to contact them?

• r/legoRockets

If the casual spaceflight and Lego userbase of Lemmy could be grown, this community would arise naturally :)

• r/Penderwicks

!penderwicks@literature.cafe would be the obvious choice, I just haven't gotten around to creating it yet.