A Sony alpha or Nikon mirrorless would definitely be the way to go. You can adapt virtually any lens to it without limitations. I also have a Leica M240, and you cannot use all the lenses on that camera because some lenses have a very large back glass element that hits and can damage the rangefinder mechanism of the Leica. With the Sony, there are no items between the lens and the sensor.
If you have the money though, go for a A7version3 at least. I hear they have a much better battery life. I went with a A7version2, and battery life is quite low. Nothing compared to my Leica M240. I can use the Leica for months at a time taking a few shots here and there. With the Sony, you can go through a whole battery in one session easily. I haven't fully tested how many shots, but if you go to for example the beach with the family for a few hours, you will want at least 2 batteries.