Need a "decimal" datatype.

[AwesomeDolphin]

This is, admittedly, not a C datatype, but many of the 65xx targets have a decimal mode in hardware, and it would be nice to have type-safe access to it.

[hth313]

It is unlikely I will find time for such feature in C.

You can accomplish it to some degree with inline assembly and macros.
A better way would be to have a C++ wrapper which would allow operator overloading and type safety. However, I have not yet enabled C++.