Ive poked around Atmel's site a bit further, and thi largest chip in the ATMEGA168's class is the ATMEGA3250. It has a similar internal structure to the 168and should be prodded into functioning with the same software (relatively) easily.
You asked about I/O pins? There's a whopping 69 of them, and I'm still reading the 375 page datasheet for further details, like
. There are (if I'm reading this correctly) four PWM-ready pins, with more available through some witchcraft.
In terms of on-board memory, it offers up 32K of flash memory, 1K of EEPROM, and 2K of SRAM.
I'll definitely want to push out all of it's pins, not for my own purposes (not sure I'll ever need that many), but for others who may want a try at maxing even this guy out.
I may try short ribbon cables leading out to individual male pins, to allow people to pull them out of a breadboard sequentially, so as not to strain the entire board by lifting all pins at once, connected to the same board. I might put on some female headers, and group similar pins to the Arduino's in the same spots relative to each other, so that all your old shields will still fit, with some new programming of course.
If anybody can think of any features they'd want put them up. This project will probably take a while to complete, so I'll take my time adding boukou features.
Here's the 3250P's datasheet for some light, 325 page bedside reading. http://www.atmel.com/dyn/resources/prod_documents/doc8023.pdf