Flow batteries seem very promising, but the chemistry required needs more scale/external funding to be viable.

There were some thermal battery retrofits for coal power plants using carbon and steam that looked interesting in principal, though cost and logistics are not fully solved problems, and the round trip efficiency was rather bad compared to other storage methods.

There were also some molten metal batteries that have been working towards useful scale over the past decade or so. They had cheap and abundant matetial inputs and significantly long charge discharge.

There are many neat options out there. I think researching and building out each as they become viable would help to improve system resiliency and long term viability.