The designated hitter was instituted in the American League in 1973. The rule, that lets a position player hit in place of the pitcher, was the brainchild of notorious Chicago White Sox owner Bill Veeck.*
If the Designated Hitter comes to the National League in 2017, I predict position players will be allowed to re-enter games by 2020. If those things become reality, the active roster limit should remain at 25 players to keep some strategy in the game.
*Note: This blog post contains information from Wikipedia that was used to confirm the spelling of Bill Veek's last name. Please see the link below: