The upgrade to doctrine/doctrine-bundle 2.0 is not exactly trivial, but I don't think it should be a major issue. (At the same time supporting ^3.0 is trivial, so you may add that as well if you manage ^2.0).
CmsBundle 4 is not out yet, and this is a pretty major hold-back in dependencies. Most notably this means anyone using CmsBundle has a forced dependency on some abandoned zendframework/* packages.
Can you please look into it? I assume that if there are any changes needed they can be backported from CmsBundle 4.