Could you help me out?

Can we say 'the thing(s) that' means 'what'? So 'that's the thing that counts' can be rewritten like 'that's what counts'? Then how about 'what is coming'? In some case, I think we can see it as the thing that is coming. What do you think? Thank you so much in advance.

Cf. Keep your mind off what is coming.

And here in this sentence, 'what happened?' we can consider 'what' as a subject itself, but in 'what happens is~', 'what' means 'the thing that'. What do you think? Thank you.