Hi Thea;
Welcome to RFF. You have some fine lenses already. Are you looking for something different than what you have? If so, then I'd recommend trying a very wide lens or a lens in a FL you already have but with a faster aperature in order to try out those lens signatures. xray is rightfully high on the ZM lenses; the ZM 28/2.8 is strangely becoming one of my regular use street lenses on my M2, just using the entire VF to frame. I thnk Roland for recommending the lens to me. Or, for a still wider POV, you may like the VC 15/4.5; I am just starting to enjoy using this lens, as it has opened up new vistas to a crusty old 5cm FL lover. But that said, as you claim to have a fondness for the 3.5 cm FL, I would recommend the VC 35/1.2. It has a gorgeous signature, the OOF areas are delicious and creamy; in addition you can shoot the beast off in candle light without a flash.
Have a blast making your decision!