In my experience, the Deco M9 Plus V2 is almost always below 50% CPU and Memory use. That means it should have more than enough overhead for some more experimental advanced features.

The one feature I feel would take this good mesh router to the next level is Advanced DNS filtering. I wish I could block every *.zip and *.mov domains, for example, since some less tech-savvy users could be fooled by this new TLD, mistaking them for file extensions. I don't see a reason not to block every single one, but the firewall, parental controls, and malicious filter provided simply don't have options enough for this.

A Pi-hole would do the trick, but since the Deco has so much spare capacity, this could be an interesting project for Deco lab. Most of the code needed is open-source in projects like Pi-hole (CC-By-SA 4.0), so adapting would be mostly designing an user interface for advaced users, as expected from Lab's users.