Hmm. I feel like 'I'll have a think about it' and 'I'll have to have a think about it' are idioms themselves (specificially 'have a think about it'). Trying to use it in novel sentences:

The opposite of the idiom - don't have a think about it - eh...
I wouldn't waste a think on him
It's worth a think (this works, it seems)

Maybe it hasn't been phased out. Maybe it's being phased out; it does seem like most usage is idiomatic. Maybe it isn't being phased out at all.

Glad I could clarify that for you.

If you think the stats are going to stay like that when the current young generation starts writing I'm afraid you've got another thing coming.

In context:

If you think that, you've got another thing coming.

Another thing = not 'that' (that's the logic of the sentence). Even said without 'if you think that...' it is usually contextually assumed.

If you think that will happen, you've got another thing coming.
Something else will happen.

If you think that will happen, you've got another think coming.
You will think of it in a different way.

The old version seems applicable in less situations and uses a word which has been phased out of the language in that form. I think it's quite an attractive phrase though because it's like a prod in the right direction: 'you need to rethink this' or 'you're about to realise your mistake'.
Still, both work, and it seems very likely that 'thing' will almost completely replace 'think', because children know 'thing' as a noun and 'think' as a verb, so will automatically hear it as thing.