It's never going to have identical features as Bitcoin Core - that was never the point.

There are some BC features that will likely never be implemented (e.g. bloom filters)
And there are some original Gocoin features the BC will likely never implement (e.g. trusted peers)

At the other hand, there are some feature BC has that are still supposed to be added (e.g. IP v6 support), although there is no roadmap for them.

The mining API is certainly a big missing feature, but since there is nobody interested in mining with Gocoin, it's pointless to add and then maintain it.