Outside of Glasgow and based on size of the potential crowd it would have to be one of the Edinburgh stadiums or Rugby park (latter of course would be even worse than Hampden)
Once you get away from them it would be either Tanndaice (14,209), Fir Park (13,742).
Perth would be nice but the capacity hinders it (10,673). Last time a League Cup semi final was held outside of Glasgow or Edinburgh was in February 2000 when a certain Arild Stavrum scored the winner at Dens Park with the dons allocation being just over 5000.