A modified tree that drops nuts of (nearly) pure graphite or some other form of nondecomposable carbon which we can easily harvest and sequester.
Photosynthesis in humans to supplement our energy consumption. Shoutout to Knights of Sidonia.
I understand the ethical issues surrounding the second one.
To match the IEAs projections for carbon capture (7.5 gigatonnes CO2 captured per year in 2050) you would need around 3*10^12 trees. That's about the same as the total number of trees in the world. So you need to replace all of the worlds trees, with the corresponding destruction of many of the biggest ecosystems on earth.
An added benefit would be if the wood after harvested lasted longer than a cedar.
I'm guessing the reason this isn't possible is more due to the fact that it's just not financially worth the investment than it being too difficult.
But I'd love to hear from someone who knows more about this stuff.
Well, we sort of have that in the form of specifically engineering bacteria to create some drugs, notably insulin. Though I’m sure graphene require additional biological structure and that is a much harder problem them adding/removing a few genes.