I always preferred the US working title, 'Indiana Jones and the City of the Gods'. Like 'Temple of Doom', it refers to the key destination of the film without fully revealing it in the title (as 'Kingdom of the Crystal Skull' does). Also, it doesn't give away the McGuffin of the film in the title (as 'Indiana Jones and the Crystal Skull' does) and is not unwieldly, and easy to remember - 'City' and 'Gods' are terms we know and understand, and can latch onto. 'Crystal Skull' is a bit out there, and probably why 'Kingdom of the Crystal Skull' doesn't work (although I suspect this title is more in line with the B-Movie Sci Fi film vibe they were going for).

This could also be shortened further if desired, by omitting the 'the', so that it is just 'Indiana Jones and the City of Gods'