If starting from scratch is unacceptable -- what microcontroller was used in the first series and why is it now unacceptable?
Why must the MCU be 16 or 32-bit?
Although I have utilize all three of the previous recommendations in my designs, according to the first line of your requirements only the MSP430 and SiLabs ARMs are acceptable, due to the fact the other devices are 8-bit.
Before a MCU can be properly selected for a project, full details of the project and its requirements must be known. Therefore you would need to elaborate quite a bit on your project and what it entails, before I would make any strong suggestions or recommendations, that being said based on the requirements stated above, you may consider the following options.
Assuming the MCU must indeed be 16 or 32-bit considering the following which fulfill your other stated requirements:
As JS suggested, TI MSP430 Family, particularly the "Value Line" Series - 16-bit available in small low pin count packages
Microchip PIC24 - 16-bit available in small low pin count packages
Microchip dsPIC - 16-bit with enhanced digital signal processing features available in small low pin count packages
Microchip PIC32 - 32-bit MIPS based design available in small low pin count packages
ARM Cortex-M0 family offered by several manufactures
For Example:
Of course if an 8-bit is a possible choice, that opens the door for a multitude of other possible options.
BigDog