Yes - we would love to build our own terminal one day. I agree, a first party experience will make Fig much better. We think a good terminal needs to be fast, cross platform, customisable, and extendable (this is easier said than done).
That said, one day we hope to build team-based tools too. Developers like to choose their tools - we would love to make Fig work, even if you weren't using a Fig terminal. We likely will also do the hard work of integrating with the existing terminals. You choose your tools and Fig is an integration on top.
If you're interested, here are some of the things we plan on expanding into. https://fig.io/videos/old-sizzle-reel.mp4