If the recent past is any guide:
- the C language will remain mostly stable and unchanged
- The C++ language will adopt many new things, in 20 years it will probably be from 2 to 10 times as complex as it is now
Some alternative programming languages will rise for the domain that is now dominated by C and C++, but they will not get much traction. The successful ideas from these languages will be incorporated in C++ (and a few unsuccesful ones too), but will be ignored by C.
C will remain dominant in Electrical Engineering curricula and careers (and in the Linux kernel). C++ will become dominant in low-level/high-performance/resource-constrained programming that is not intimately tied to electronics.
Hardware will continue to evolve, hence things that are now done in C and C++ will be done in other, more programmer-friendly but less performant (less CPU-friendly) languages. New application areas will arise that require performant languages to get the most out of the hardware, these things (gadgets? wearables? intelligent dust? who knows) will be programmed in C or C++.