I'm planning to make (and sell) a shield for the ever-popular Wemos D1 Mini ESP8266 board, which will consist on an ATtiny (I'm thinking a 1614 or 3216 if I can fit it), with serial pins connected to those on the ESP8266,
The idea would be that you'd do all the internetty stuff on the ESP8266 (probably running Espruino, because doing that is really easy on Espruino compared to doing it in C). Then it would communicate with the ATtiny over serial, passing commands/data/etc back and forth. In that way, you'd have the easy web interface from the ESP8266 - but also a set of decent peripherals with multiple good PWM channels, input capture, a DAC, decent ADC channels and lots of them, and so on. ATtiny would be programmed in Arduino C.
Judging interest here - would anyone else buy something like this, or should I just build what I want and not plan to sell them?
One use case (the one that I have currently in operation without a custom board which inspired this) has the tiny connected to a 433MHz RF receiver and transmitter, running my in-house AzzyRF protocol, and it sends all packets received via serial, and can accept a serial command to send a packet. Meanwhile, the ESP8266 keeps a history of the most recent 20 commands and when they were recorded and can return that in response to a GET request, and likewise, a different get request will send a packet specified as a URL parameter. In other words, it's a WiFi to RF bridge. I'm already using it (along with my fauxmo script that provides glue between Alexa and other devices) to control an RF relay box I made which in turn switches on and off two lights, and a dab rig, and turns them off if they've been left on for too long. The ESP8266 side, upon notice of a received packet, can also match it against a list of "key" packets and take actions in response to those (for example, I plan to make it update the external webpage that tracks state of sensors in my room).
Whether I'm building for myself only or others actually makes a difference, because if I'm doing it for myself, I would build it specifically for the above-described WiFi-RF bridge - IE, on-board antennae and dedicated spots on the board to mount the transmitter and receiver modules, and decisions on layout optimized for that application, rather than for general purpose.