I wouldn't mind seeing a version of Batman Beyond
that's a continuation of the Adam West series. I'm sure West would jump at the opportunity.
I like this idea of a period Batman. If I were doing a retro Batmobile I don't think you need to adapt an existing car. Maybe I'd start with something like a '48 or '49. Cadillac as a template and work from there. I could envision something of a bat like symbol incorporated into the front end and bat fins integrated into the rear fenders without getting too outlandish.
I'd probably give him a regular-looking car that has a lot of hidden features and weaponry, similar to the Green Hornet's car or KITT.