If you're looking for a "successor" for the ESP8266, then it's clearly the ESP8684, part of the C2 line - even Espressif says so.

The ESP32-C is a RISC-V, unlike the others.

All the new ESP32 parts are RISC-V

Isn't the C3 the only one so far?

ESP32-C2, ESP32-C5, ESP32-C6, ESP32-H2, ESP32-H4 and ESP32-P4 are also RISC-V. Xtensa are ESP8266, ESP32, ESP32-S2 and ESP32-S3.