Any other new soft fork in the works?
What are the upcoming scaling and privacy improvements for Bitcoin past 2021?
Is Bitcoin done with improvements yet?
There will always be scope for improvement in a software. Since Bitcoin is a protocol for decentralized network that allows you to settle payments, neither 12 years are enough for development of such network which is trying to change how money works in this world nor we have explored and improved everything. Infact lot of basic things are still being developed.
At some point, almost all new development should be done in sidechains instead of in Bitcoin, right? There's already more development in sidechains than in Bitcoin.
Sidechains development and usage can work with on-chain development. Liquid and Rootstock exist as sidechains right now but I am waiting for better and more sidechains or drivechain project to be used in future. Also Lightning Network can't be ignored which is first choice as layer 2 solution for many users and developers.