Scaling up carbon removal and other promising climate-related technologies before governments are willing to fund them. A lot like what Stripe and Shopify have been doing, but about an order of magnitude bigger. If the timing is right (I'm not sure it is) this strategy could get a fair bit of leverage by driving costs down and accelerating even larger-scale deployments.
A few people have mentioned retroactive public goods funding. I'd suggest broadening the scope a bit: