Gary makes a come back in Sinnoh, now since Ash beat him he has quit being a trainer and is working for his grandfather professor Oak by protecting a herd of Shieldon.
Ritchie makes a come back in Johto and helps protect Lugia from team Rocket (not the comedy relief one the actual organization)
none of his other rivals are even seen again.
Edit-Gary was also in Johto but he was still Ash's main rival so it technically doesn't count as a come back