Greetings from the Custom Hearthstone side of the web.
I must ask why you prefer to make mechanics that work for only one expansion when you could stretch the idea out to work all year in three? I liked how Treant Druid was spread out and wouldn't have minded that for Freeze Shaman as an attempt.
We do a mix of this. Sometimes a mechanic like Rush and Lifesteal and Discover is kept for years. Sometimes we do it for one expansion. Freeze is a good example because it isn't very fun to have your minions frozen a lot so making that into a Tier 1 deck is pretty dangerous. We have seen this in mage already.
When we have a build-around like treants or elementals or dragons we can launch it and see how it goes, then slowly add cards to increase the power level over the next few years. Dragon Priest is a great example of this because it has been a reasonable deck for years and it changes each year as some of the strong cards leave, and new strong cards show up.
In the future we want to have more decks that rely on cards in two different years so that when rotation happens the deck changes, with some cards leaving and new cards entering the decks.