UART, I2C and PWM are already supported. Since UART and I2C are much simpler and also very restricted on what PINs you must use, start from there. PWM works but has limitations of which the main one is that you cannot share timers. If you are using TIM0 for the ticker you'll only have TIM1 or TIM2 (I don't remember how many are on the 328p). For INT, an ext driver was contributed by Igor and is available on his Github. As soon as I get some free time I'll try to incorporate it, which could happen before you need it, maybe. You can also do the manual programming of INTs which is not that hard.
If you change/fix issues in drivers, we'll be happy to get your feedback/patches.