A rematch with Pascal would be good but I don't see the Shark beating him. Some guys just have your number and Pascal has his due to styles. I know this is a longshot but hear me out. Zsolt Erdei fights unthreatening opponents and others' leftovers (Sahnoune, Barashian, Ulrich). If GYM could make the case that Diaconu isn't a real threat to Erdei's title surely they could at least have some talks with Universum about a potential fight between the two. They can use Diaconu's last two fights as proof that while he's a decent fighter, he's very beatable and his would be a recognizable name to add to Erdei's win column. Location wouldn't be a problem as Diaconu has no leverage so Germany would naturally be the site. Erdei would be the favorite but Diaconu would be a live dog. I know that it's highly unlikely but if we're talking title shots here this is Diaconu's best chance.
Erdei would never fight an actual threat like Diaconu, who while limited can still dent that chin. I would love to see Erdei get knocked out but it will never happen. There are actually some people out there that consider him the true champion at 175... if you follow it by lineage, he actually is...
I know that it's unlikely but it's not as though undefeated Europeans haven't tested the waters at the tail end of their careers. Grigorian did it against Freitas to name one. Erdei just has no desire other than to be a professional fighter. That's why it would be up to GYM to make the case that a fight with Diaconu wouldn't be anything more than a routine defense for Erdei and that is not totally impossible. Just my opinion. In addition, you are right about Erdei being the linear champ although that title has been watered down to nothingmore than a footnote. Diaconu doesn't exactly have a ton of options outside of a Pascal rematch because of his limitations as a fighter. He's always gonna be the shorter fighter and if the opponent can box a little and withstand an assault or two he's looking at gatekeeper status in the near future.