And how do you decide which ideas and therefore which play styles you wish to block?
Take colonisation. You'd unlock the Exploration idea group - but a 'new' version would lose the 2nd extra colonist (the fifth bonus), replaced with something else (maybe lower native aggression or something). If you want to really go for it and get Expansion (which lets be honest, is usually the 2nd idea colonialists always go doe), then you would first have to unlock something else to go alongside it. Perhaps maritime or naval - or if you were a land coloniser, administrative.
So you couldn't just go for expansion straight away. Basically, at the moment, if you are close to the Atlantic and unlock exploration and expansion, you are well on the way to colonising half the world before other nations get a look in. This is a means to slow things down a bit - but still allow you to focus on it if you want. The focus is more expensive.
It's just an idea. It would need a reworking of idea groups - possibly a few more. But it allows for you to customise your nation a lot more than before.
Espionage is an idea that could really come into play. Reducing idea or tech cost by 20% if you have a Spy (maybe you have up to 2 of them) placed in a nation that is more advanced in certain areas.