A great idea, but it could cause some issues with bans since some would be very strong as permanent semi-evos. Then the question is do you ban the character entirely or put in a sort of stage check to only ban semi-evos and allow full xp versions.
Having it cost clintz is a great idea though because it would be a nice way to get clintz out of the market and help stem inflation. Especially if you have to pay to evo-lock on each copy of the card and is non transferable, it would do wonders for the health of the game economy.
But then you have cards like Dregn Mt, where his semi-evo is a staple, but he also costs way too much to have to buy a new one after a certain number of fights. Same idea can be argued for a card like caelus cr and many other cr and MT cards with good semi evos.