I don't think Aimar was a flop. He was very good for Valencia untill..*cough* *cough*..Ranieri came and figured Fiore would be a better choice. That and injuries are the reasons his career went downhill, even though at the time he had already proven himself IMO. He won 2 titles, UEFA Cup, runner up in the CL.
Thing with Modric is, he's proven himself on the International stage and I believe when you do what he did, at such a young age, there's little left to accomplish. Club based success will come to him I'm certain, he's mature, level-headed and immensly talented.
The only scenario in which Modric would flop IMO would be some series of horrible injuries. But that won't happen.