Sorry about the meta-like post without any card designs myself:
In reading this question, I wonder whether the prompt is talking about skill in deckbuilding, in in-game play, or both. I think it is challenging to create cards that will clearly do both, but it is a reasonable goal, and I would love to see how designers think their creations would address these considerations.
Personally, I think that bolstering enchantments as a permanent type could open up space in the metagame. This could come from a toolbox/opalescence planeswalker as well as additional printings. And by shifting the metagame (were such a strategy to become feasible), sideboard and deck construction would have to adapt, making in-game sequencing and permanent removal more of a consideration.
Unrelated, I would love to see a card that requires players have at least as many lands in play to cast a spell, for many of the same reasons as above.
Last, a mechanic that gave an effect/put a permanent directly into play without the stack (similar to forecast, without the resuability) would be interesting (if a slippery slope), since it would dodge both sphere effects and counters.
Just some thoughts. Thanks for the thread!