Known for banging Barbara Wright, Ian traveled with her, the Doctor, Susan Foreskin and Dicki through time and space in the search for the missing episodes stolen by Ian Levine. He was knighted for his efforts, ironically in an episode that's still missing.
Fans have been clambering for his return ever since Coal Hill School briefly appeared again in The Day of the Doctor and became the center of the universe for Clara Who, and to be honest it's baffling that he didn't show up. Like seriously, he was confirmed as still working there on the billboard outside the school - you'd think with the rate that Capaldick kept landing the TARDIS in the classrooms that Ian would've belted for whichever room the engine noises came from, burst in and yelled "AWH YEA! IT'S MAH NIGGA, THE DOKTAH!"
Although in all fairness this hypothetical return would literally be a copy and paste of School Reunion, and let's be honest here; School Reunion is more GOAT than (almost) anything in the Capaldi era, and would be an incredibly tough act to follow. Best not to attempt it with the Moffat writing team; leave it to Big Finish, after all they handled Susan's return very well.