But if you want something really sophisticated, use an interrupt controller, such as the 8259 (used also in PCs - today integrated in chipsets).
"sophisticated" ?? how about "can be used if you bend, fold and mutilate your hardware and software"
Or at least, get inspired by its complexity.
there you go

anyhow, here we go:
I assume you mean add "external interrupts"
set a timer to ffff and configure it as a counter you get an interrupt on the first pulse on TxIN
if your processor has a PCA, you can "fabricate" 5 external interrupts
if your processor has comparators, the can be used as logical interrupts
if your process or has an A/D with a limit interrupt, again useable
I'm sure there is more, this is what comes to mind

BUT, if you say "I want to stay with my little basic chip" you are stuck or will need some fancy logic, and that is the crux: why add "fancy" logic when another uC e.g. one with comparators will do the trick?

Erik