According to this report Tim Roemer, the former 2nd CD congressman, is considering a run against Lugar next year. This would be an interesting matchup, like Lugar, Roemer has made his name on Foreign affairs issues, specifically his role on the 9/11 Commission. Also like Lugar Roemer is a thoughtful moderate (words you'll never hear connected to Chocola's name) who can work across party lines.
There is a very early poll that shows Roemer trailing within the margin of error at 41%-39%. Given that Roemer is cut from much of the same cloth as Bayh, he would seem like a very serious challenger to Lugar. And there is a history of Hoosiers dumping long term Senators, Vance Hartke and Birch Bayh being two examples, but right now he faces what must be a fundraising deficit of Everest proportions. The likelihood of Roemer even entering this race let alone having any chance of winning will be tied to his ability to catch up.