Very cool. This sounds like an awesome project and thanks for sharing! Among the various applications, this one — to compute Evotraspiration ET0 using WU stations data or local sensors and to adjust Open Sprinkler rain delay — is a particularly interesting approach, as it allows ET-based weather algorithm without having to upgrade OpenSprinkler’s firmware.
As I’ve started using ESP8266 a lot myself, I really think it’s a fantastic chip, very low cost, easy to use, and have excellent community support. The lack of GPIO pins can be easily solved using an IO expander as you said; I am not too worried about the lack of ADC pins because a lot of modern sensors are I2C-based any ways. Probably the only thing that stops it from being used absolutely everywhere is the power requirement — a WiFi-chip is still quite power hungry and that makes it less suitable for certain projects that need to be placed outdoors or in places where there is no persistent power.