What could we expect from future innovation to improve Graphene OS security ?
I'm thinking about the recent MTE improvement, for instance.

I have no idea about hardware security improvements akin to MTE, but I do know of a few planned software improvements.

The first that is coming soon is increasing use of virtual machines, which will allow for greater sandboxing of apps than is currently allowed.

https://xcancel.com/GrapheneOS/status/1900724809379528905#m

Another that is planned is more scopes, like the storage and contact scopes. Specifically camera, microphone, location, and possibly app communication.

https://xcancel.com/GrapheneOS/status/1889015093456678973#m

In the long term, GrapheneOS aims to move beyond a hardened fork of the Android Open Source Project. Achieving the goals requires moving away from relying on the Linux kernel as the core of the OS and foundation of the security model. It needs to move towards a microkernel-based model with a Linux compatibility layer, with many stepping stones leading towards that goal including adopting virtualization-based isolation.

https://grapheneos.org/faq#roadmap

I'm sure there are others, but those are some major planned software improvements I know about.