IIRC Exanima itself was never meant to be the full open world RPG. It was always intended to be a smaller game to perfect some of the game mechanics for their ultimate goal of building that open world RPG. I have no idea if they still plan to build that other game or if they are working on it in parallel or have ditched it entirely.

Edit: The community seems to believe that the devs are still planning to make Sui Generis at any rate. Exanima has been in EA for 10 years or so now, and based on what I'm seeing online they are almost at their 1.0 release version, at which point they will divert their attention to to Sui Generis. Take with a pinch of salt, as this information comes from the r/exanima community on reddit.

Yup.
Sui Generis is being worked on simultaneously. Anything developed for Exanima is used in Sui Generis.

It's one of my favourite games despite not being finished yet, but it will probably gain more popularity once we get co-op and a multiplayer arena mode.