Collocations for unthinkable - adj.

be, seem
make sth
consider sth
This would have been considered unthinkable only a decade ago.

absolutely, completely, literally, quite, simply
almost, nearly (esp. AmE), practically, virtually
hitherto, previously
A hitherto unthinkable question was asked.

politically

for
It's unthinkable for something like this to happen.

to
This course of action would have been unthinkable to the generals.