wp have all versions like 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, etc (and 1.2.x minor updates)
I think it is good idea to use similar versioning.
If next release after 1.1 is big update, it is not necesarry to bump version to 1.5...
If next release after 1.8 is big update, it is not necesarry to bump version to 2.0...
It MAY be OK to bump version if there is complete rewrite AND breaks compatibility with plugins, but it not essential even in this case.
I think it is good idea to use similar versioning.
If next release after 1.1 is big update, it is not necesarry to bump version to 1.5...
If next release after 1.8 is big update, it is not necesarry to bump version to 2.0...
It MAY be OK to bump version if there is complete rewrite AND breaks compatibility with plugins, but it not essential even in this case.