Well, actually… it wasn’t obvious to me whether OP was supposed to be interested in clocked local protocols like I2C and SPI, or whether they were supposed to be looking at the higher level “synchronous communications protocols” like HDLC or BiSync.
I hope for their sake that it’s the former, since afaik there are no Arduinos that support the continuous clocking and bit stuffing required by the latter. But a bit-banged HDLC would be an “interesting” project (presumably at a pretty low speed…)