I won't be surprised if he churns through a bunch of different ideas: Subscriptions, Deals with news companies, short form videos + creator tools, different kinds of ad experiences, the edit button, better spam filtering etc.
There's a lot of potential things Twitter could be doing if they didn't limit themselves to their core product.
I don't think (b) is unlikely. Their current product is already highly valuable. You don't need their short form video product to succeed in a way TikTok did but there's enough of an audience to capture there to improve the company's valuation.