Short-haul and medium-haul and long-haul conventional take-off electric aircraft.
There's a lot of work on VTOL electric aircraft and hybrid aircraft. There's a lot of capturing low-hanging fruit by trying to electrify existing airframes. On the higher end, there's a lot of work on enabling lithium-air or lithium-sulfur or solid state lithium or metal electrode lithium ion batteries. However, there's not a lot of work on combining extremely high efficiency airframes (i.e. glider-like) with extremely high performance batteries (and high battery mass fraction, i.e. 60%, not 25%). Often one or the other is the focus, but a combination is essential to enable decent range (1000km) with existing batteries and long-haul range (at 4000-10,000km) with advanced chemistries (lithium air).
Another issue is low capital cost hydrogen production. There has been a lot of work on reducing the cost-per-watt of solar and wind power and fuel cells, but progress on cost-per-watt on hydrogen production (i.e. electrolysis) has been lacking. That means it's not financially viable to produce hydrogen from excess intermittent renewable energy, in spite of the clear potential (if the cost-per-watt of your electrolysis equipment is much higher than the cost-per-watt of wind or solar, you need to run near constantly to make your capital investment make sense... hydrogen doesn't work as a flexible demand source if the capital cost is so high the plant has to run all the time).
Microbiome modification to reduce methane emissions from ruminants (cows and sheep and goats).
Liquid hydrogen powered container ships or long-haul aircraft (and a method of dealing with the water emissions so as not to release water vapor at high altitudes).
High efficiency deep cryogenic liquefaction of hydrogen.
Battery-electric long haul locomotives (perhaps with intermittent charging via catenary).
Extremely cheap Level 1 or 2 electric charging outlets. Literally an outdoor outlet with a relay controlled by an app that can be deployed at just ~$30 extra beyond a regular outdoor outlet. Could enable mass deployment of electric car infrastructure at very little cost (possibly even self-funding due to a small fee).
Carbonfree (or carbon negative) steel, aluminum, and cement production.