Make Sailfish OS run on Nokia Devices.
I think sticking to pure android, and of course, good play store apps, is what most users want. That will be essential to HMD Global's financial success, in my opinion.
That said, on the desktop, I'm a Linux guy. I'd rather Nokia just unlock the bootloader and be helpful, rather than obstructionist, to community supported independent open source projects that want to create interesting alternatives for those that want something different on quality Nokia hardware. Additionally, once HMD Global and Google stop supporting models as they age, open source projects can provide a second life to older hardware, at no or extremely limited cost to either HMD Global or Google. This is a far superior end-of-life alternative to running an old, unpatched, out of date operating system. Ultimately, this creates value to HMD in the form of increased demand for Nokia phones due to their longevity, flexibility, and community minded spirit. These phones will likely have a hardware life far in excess of the 3 years that they will get official security updates for. Being supportive of open source is a win-win for both the company and the phone owners.