I thought they had Y and Y+ already.
The first half of MD11 Y cabin has wider seats than the rear half and the A340's have different seat pitch in the front half compared to the rear half of the economy cabin.
They partly control this by blocking seats in certain economy booking classes, but of course you can change seats once on the plane. The seat pitch part on the A340 is not that obvious even when you're onboard .. a tape measure would be useful

And then there are the business class seats in A340 row 6/7 that they sometimes use for economy passengers in a kind of free upgrade - you still get an economy meal and service though.

Not that finnair staff will tell you any of the above and none of it is publicized so you can easily downgrade yourself from nice seat -> bad seat even if you qualify for one of the nice seats - since how are you to know which are the nice seats without some research.