I like the simplicity but worry it might be an over-simplification.
The significance score used for ranking is more subtle than actual / expected because that has a big bias towards very rare terms. Rare terms can easily experience relatively big jumps in stats (e.g. expected =1 vs actual 2) and more common words cannot experience those same levels of growth as easily. It's like how fleas can jump much higher than elephants (relatively speaking). So in the significance score I use the magnitude of the jump is tempered by a sense of scale.

I'd like to see the actual significance-score used for ranking retained in here somewhere otherwise the "expected vs actual" score you propose does not actually match the sort order/selection criteria used behind the scenes. 
"Expected" ("bg_count?")  is a derived number and a bit of a guess, whereas supplying the background frequency feels like you are giving clients more reliable stats from which they can draw their own expectations.