Well, actually, the idea of Clint as Batman has been thrown around before, as I recall. After Batman Returns was done, and Clint Eastwood had asserted himself as a great director and actor with Unforgiven, there was talk that Tim Burton wanted to do an adaptation of DKR with Clint as the ageing Batman - As it was, that would've ended Burton's trilogy of Batfilms nicely, because with a little work, you could make DKR a direct continuation of Batman Returns. As it was, it never came to fruition and we got two Schumacher movies instead.
But yeah, with a little work, it's not too difficult to imagine a 'Gran Torino' style Clint Eastwood as Batman Beyond's Bruce Wayne, given thematically, they are really quite similar.