Thoughts on the expression 腦洞大開?
Coderzelf's point about the origin of 脑洞 is correct (At least 脑洞 comes from 脑补 is the most accepted view) , but his explanation for 脑补 isn't so accurate. Though sometimes 脑补 is used to express "figure out the actual meaning of something very implicit", more frequently, this phrase is used for deliberate delusions. 

Another possible origin of 脑洞 is it comes from the phrase 脑子进水, which means… er … "out of one's mind", so 脑洞大开 is sometimes used to express that something is foolish, but notice that this is mainly joking, not ironic.

BTW, 脑洞, 脑补, etc. are all new phrases and their meaning isn't completely stable currently, So their meaning may changed in the future. (Does anyone have the newest edition of 《现代汉语词典》 at hand? Has it included these phrases? They are included in some internet encyclopedias though. ) Here's a reference for 脑洞:

http://zh.moegirl.org/%E8%84%91%E6%B4%9E



One thing I forgot to mention: Actually there's a tendency to use 脑洞大开 as a joking version of 异想天开. This isn't mentioned even in any internet encyclopedia, but it's true  as far as I can see.