IMHO, the Lineage OS port shouldn’t be considered an option until it is fully official (i.e. included in the official Lineage OS build roster).
On the other hand, I think FP Open should fix the regressions Google started to introduce with Marshmallow which stand in the way of UnifiedNLP, microG and other GMS alternatives, and that could be the strong point of the OS over Lineage OS. Namely:
- Include the option to spoof signatures without the need to patch the system every month.
- Code patches: https://github.com/microg/android_packages_apps_GmsCore/tree/master/patches
- @larma’s standpoint about how to implement this securely: https://github.com/microg/android_packages_apps_GmsCore/issues/336#issuecomment-284036039
- Fix the regression of registering network location providers installed as user apps (not system)
- (I think this is Android 7, but I’m not sure now) Fix the regression of limits for alternative calendar and contact sync adapters (e.j. DAVdroid)
- Detailed issue: https://forums.bitfire.at/topic/1334/davdroid-1-4-release-notes (second bullet point)
(I’ll add links and code patches later, I’m in the go ATM) (this could be seen as the successor of the great but now abandoned BRNmod ROM)
Next, when FP2 gets the status of official Lineage OS device, maybe Fairphone should base the FP Open OS over the Lineage OS codebase, but applying the above patches (since Lineage OS doesn’t support them officially and CyanogenMod repeatedly denied them in the past).
That would be the only official Android 7 the FP2 could get with the current situation*.
P.S.: at the moment, Fairphone stills pushing Qualcomm to implement missing features (Vulcan?) for the MSM8974 SoC to be able to ship Android 7 with GMS for the FP2.