Those ARE big shoes to fill, and it really takes someone who has quick wit, and is able to really ad-lib and improvise on the spot, while remaining in-character the entire time. Some performers have been able to pull it off really well - Bill Barretta for example, some of Pepe's personal and live appearances have been hilarious; while others have had to take time with it - looking at some of Eric's earlier appearances, he seemed stumble a little, but in more recent years, he's really been on the ball.
Kevin has always been top-notch when it comes to Elmo making appearances, so if they don't limit or eliminate Elmo making appearances altogether, then whoever the next performer is is really going to have quite a challenge ahead of him.