Realistic
Batteries. It's holding back a LOT of things. A lot of technologies are solved, but just require power.
Semi-Realistic
Room Temperature Super-Conductor.
If that can be solved, the power density and efficiencies would just be astronomical....
It would absolutely destroy multi-billion industries overnight.
Way-Out-There-Stuff
If they ever prove out an actual functional EmDrive-like thruster, that would absolutely open up space travel to our species.
This could theoretically be done with stemcell stuff, but it's not there yet. However, when we finally reach the point where we can infinitely regenerate our body cells, we'll become effectively "ammortal"; unable to die due to natural causes (such as illness), but we will still die from other people (for example, a bullet to the head)
Besides that, I think nuclear fusion would be an incredible development if we can finally harness it to power our homes.
Reusable rocketry, specifically SpaceX Starship. If it pans out it's going to completely change our access to space and make many of those old dreams from the 1970s plausible.
RNA vaccines for basically everything, including customized vaccines for cancer. There's also actual progress happening in general cures for autoimmune diseases.
Is robotics too close to AI? There are multiple companies working on general-purpose humanoid robots intended for mass production with price targets in the ten to twenty thousand dollar range, we may be getting within sight of actual robot butlers.
Internal alpha-therapy.
Imagine attaching a radioactive atom to an antibody that would fix to a cancer, then as alpha radiation do a lot of damage, at a very short range destroy the cancer without doing much damage to the rest of the body
The democratization of embedded programming and the capacities it offers. Coupled with 3D printing you can build your own robots or machines with minimal knowledge and money.
I always find stories about carbon emissions exciting. The reason why a Malthusian view of things hasn't panned out in recent centuries is because of technology. I am always interested in if humans can find a breakthrough technology that basically does something with carbon emissions that could eventually be done on a scale to reduce climate change. Unlikely, but fun to imagine.
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.
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.
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.