Do sponsors expect athletes to continue delivering extras that weren't in the original contract?

Not necessarily. If you do something extra like putting logos on merchandise in year one, sponsors don't necessarily expect it to continue forever. It could become a negotiation point for the next contract. Sponsors understand that circumstances change year to year - what's possible this year may not be possible next year. If an athlete can sell that extra opportunity to someone else for money, good sponsors will tell them to do so because they understand the athlete is trying to run a business too.