I'm not saying any EU city would work. Indeed, I'm suggesting quite the opposite, that homebody Clevelanders disdain international travel. I support my thesis by pointing to squandered opportunities of yore.
I lover international travel -- but I also wouldn't sacrifice comfort and perks to fly a random airline on a small aircraft nonstop versus flying on my terms with a brief connection.
I think the systematic problem with INTL from Cleveland is that there's not an industry that drives a lot of single-market O&D -- the closest I've come from personal observation is Goodyear seems to send a lot of people to/from FRA/MUC, which means people are going to need to connect on one end or the other anyway and connecting via a US gateway city gives me the most flexibility (e.g. when CO was running LGW/LHR that was at best 1 flight a day right? Vs. what -- 30? 40? for any US city-LHR)
I vote for a nonstop CLE-ULN. That was a real chore with the double-connection CLE-ORD-PEK-ULN. Actually I wouldn't be surprised if there was a decent O&D count for CLE-AUH(/DXB?) + onward connections but not enough to support the capacity required for an aircraft (currently) capable of making the flight.
It is interesting looking at the GSA City Pair program contracted cities for 2019 -- the majority are domestic awards -- and of course only capture US government travelers on official travel but from CLE the following have been awarded:
CLE-BRE (Bermen, Germany) on DL @ 84 annual travelers,
CLE-GUM (Guam) on DL @ 164 travelers,
CLE-OKA (Okinawa, Japan) on UA @ 133 travelers,
CLE-FRA (Frankfurt, Germany) on DL @ 67 travelers
CLE-GVA (Geneva, Switzerland) on UA @ 43 travelers,
CLE-LON (London, UK) on UA @ 60 travelers,
CLE-PAR (Paris, France) on UA @ 44 travelers,
CLE-SEL (Seoul, Korea) on UA @ 43 travelers,
CLE-TYO (Tokyo, Japan) on DL @ 172 travelers
CLE-BUH (Bucharest, Romania) on UA @ 48 travelers
So even among our government travelers it's more of a shotgun spread than a laser focus that would make a nonstop flight viable.