I don't think they're fixing clay generation again. That was an old tweet from 4J before the quick fix update. Remember if you want clay you have to create a post-patch world or it wont work.
Another thing coming in the 1.7.3 update is being able to see the seed on a random generated world.
Also with the update will be a Skin Pack DLC which will contain 40+ skins, no word on the costs of DLC yet. A few of the skins will be free though.
Lighting will also be fixed in the new update, making the game brighter and fixing light not filtering into areas where a block was just mined.
Update coming Mid-July