I don't understand the obsession with making everything into trilogies.
also I'm not really a 'batman fan' I like the character but I don't know the comics and don't care to either really. I mean I'm sure some are good but I don't think the filmmaker should be locked into what is already on the page.
you should be able to take the central 'truths' of the character, some skeletal plot details etc and do with them what you like.
if I were WB I would look to make a selina kyle prequel. not a batman film, but a film set in the same city (probably in the 8 year interval between tdk and tdkr) and make it deal with the consequences of the 'harvey dent act'. you basically have the chance to make a heist film inflected with a criticism of guantanamo bay / examination of heavy handed laws etc.
you use what you have- a good character, a well drawn and established world, a waiting fanbase... it makes sense. moreso than to just go "well what happens next? lets get person x to wear the cape and fight bad guy y from ruining gotham"