My experience with miniatures games is limited to WotC SWM and X-Wing. Both suffer from power creep. The former more than the latter it would seem.
How can the designers help mitigate this?
For starters I don't want to see new versions of the current commanders. I think they should be one and done. The only exception would be if they make the Resistance into a separate faction. Old man Luke, Han, and Leia would be acceptable in my opinion. Likewise a Sir Alec Guinness Obi-Wan for the Rebels and a Ewan McGregor Obi-Wan for the Republic would make sense. But I don't want to see Luke Skywalker Farm Boy, Luke Skywalker Jedi, Luke Skywalker Pilot, etc released.
Another thing that would help mitigate power creep is to make Luke and Vader the most powerful commanders for their respective factions. Perhaps there would be commanders with better command cards (like Leia, Dodanna, Thrawn, Tarkin, etc) but no commander should be so deadly on the battlefield as they are (with the exception of a possible Palpatine, Yoda, and Obi-Wan).
But what about troops? I don't want to see stormtroopers disappear because the snow troopers out class them in every possible way. Should the stormtroopers just be the cheapest unit forever to give players incentive to continue fielding them over more powerful options?