One thing I learned watching Miko for years was, if she says she has something in planning it's because she really has a project in planning, she always gives these indirect messages when she knows of some project that will be released soon
After saw Holoearth PV, I kinda think that it is what they are talking about in this clip. Especially given Suisei don't really remember (since it is not a big project like Live/unit song/cover) and Mikochi not sure whether it can be considered small or big project (since they only appear a little) and she said that the project has been in the works since a long time ago. Just my wild assumption tho.