One set of applications to build with subscription is to use the claude-go binary directly. Humanlayer/Codelayer projects on GitHub do this. Granted those are not ideal for building a subscription based business to use oathu tokens from Claude and OpenaAI. But you can build a business by building a development env and gating other features behind paywall or just offering enterprise service for certain features like vertical AI(redpanada) offerings knowledge workers, voice based interaction(there was a YC startup here the other day doing this I think), structured outputs and workflows. There is lots to build on.