I was surprisingly unable to find any reference to this phrase here on EL&U or the internet in general, but since we seem to be pulling at straws here I will offer another possibility:
shudder to think
I'm vaguely familiar with the term, and this was the only reference I could find:
I dread/shudder to think
something that you say when you do not want to think about
something because it is too worrying or too unpleasant (usually +
He was going so fast - I dread to think what would have
happened if my brakes hadn't worked.
Apparently "Shudder to Think" is a rock band, which may have been clogging up my search results. Anyways, I'm not certain this is appropriate usage, but it sounds reasonable to me (albeit not something I would personally care to say):
You could do X if, shudder to think, Y doesn't happen.
In cases where you would use "God forbid" at the beginning of a sentence:
He can't drive. God forbid he gets behind the wheel.
...It doesn't work as well, but it still can:
He can't drive. I shudder/dread to think what would happen if he gets behind the wheel.
Quite a mouthful compared to "God forbid", but it seems this phrase is usually sandwiched between by "I/we" and "what would/could/would have happen(ed) if".
Once again, I'm not certain if the first example is proper usage or not, but if so - it seems to be a decent replacement.