@dkayiwa, Bahmni and most RA implementations should be running core 2.0.x+ already so I wouldn’t be so pessimistic, when it comes to finding first users, if only supporting core 2.0.x+.
Yet, I feel SolDevelo should base the decision on partnering with an org, who wants to use it the moment it is ready and PIH seems to be interested in deploying for 1.9.x. In the first pass I would aim to develop for the exact version you want to use it with first.
It feels that extending Sync 2.0 to support other versions of OpenMRS would be a similar effort now and after implementing it first for a specific version assuming we have solid integration tests, which can be run against different OpenMRS versions and considering some major differences now.
If you intend to use webservices.rest and fhir modules then they support 1.9.x+ and 2.0.x+ accordingly so at least fhir would have to be adjusted for older versions.
We also upgraded Hibernate in core 2.0.x, so you will need to handle small differences, if you are working at that level to discover updated objects.
There’s also a matter of UI difference. The older implementations most likely use legacy UI (jsp), whereas newer implementations use UI framework (gsp). That, however, could be easily handled by implementing UI as OWA, which should work regardless of the core version.