Unlocking any energy conversion techniques for gravitons (not virtual gravitons, but those associated with gravitational waves). If we could produce them artificially, it would be a whole new ball game.

Quantum safe cryptography ciphers already exist

Excited and scared for quantum computing

The steady improvement in computer speed and efficiency (unfortunately brought down by bloated software, but in some areas you feel it in absolute terms), storage and memory size, and EV technology. I hope in 2040 there will be cheap powerful e-scooters and e-motorcycles.

Fusion? That would be big. The continual role out of green energy which can push the price down. The McRib coming back. Normal things.

Differentiable programming. Differentiable ray tracers for example can be used to reconstruct the geometry of something you took a picture off.

That one day they will harvest my brain and let it live a vat of dopamine

Regenerative agriculture

Nuclear power reactors built after the 1970's. New generation (5?, 6?) for baseload. Mox, msr, lead moderated... Renewables can bicker over the transient loads while reactors provide the 'always on, always needed' bulk of power load.

Fuel reprocessing to close the loop would be the grail at this point.