I just killed an Elder Dragon outside Dragontooth Crater whilst on my travels through The Elder Scrolls V: Skyrim, after which a misc. quest marker popped up telling me to 'Return to Esbern'... Intrigued, I set off to Sky Haven Temple and found that Esbern had nothing more to say than; "Hmm?", "Yes?" or "Need Something?"... I'm guessing this must be another one of Skyrim's infamous bugs.