Sorry I didn't see this sooner - I thought I would get a notice of anyone posted.
What I think the next wave of add on expenses may be?
I am going to assume you are asking about "optional" add-ons, however I will address one that I think will become mandatory soon.
I think service charges will soon become mandatory industry-wide. Right now a person can technically choose not to leave gratuities. This is becoming a problem for some of the lines like Royal Caribbean (where crew members are starting to complain about the many deductions made to their tips by the company to "tip out" to different service providers on the ships.) and also by Carnival where crewmembers are complaining about how the cruise lines are changing their salary potential by changing the number of passengers they service based upon their ratings.
Optional charges - Carnival has started offering early boarding for a price, and I personally think it is worth it, especially if you have been traveling overseas, for example.
I foresee more cruise lines really pushing their drink and dinner packages. Especially drinks.
Right now many cruise lines already include the price of cruise insurance in the final price (making it so you have to take it off the final invoice if you do not want it). It might actually be a good idea for the industry to make "cruise insurance" mandatory. That way people who are denied a cruise because of a visa problem, or a death in the family, will always be covered.
They could work it like this - CLIA could recommend it to all cruise lines as a safety issue - making it "mandatory" for the cruise lines to sell cruise insurance with every cruise.
I do foresee a day when ships charge for deck chair reservations. Because it is a logical answer to "deck chair hogs" - if you want to reserve a deck chair for the day, you have top pay for it.
Cruise ships are already hiding entertainment charges by making certain shows "dinner shows" (Cirque Dreams & Dinner on Norwegian) - well, they would have served those people dinner anyway, so it is possible that someday there will be an entertainment charge onboard.
I don't know what else, the cruise lines are already charging for things I never expected - premium steaks in the dining room for example.
In Europe they have a service fee for room service. I can see that possibly becoming a new charge some day but I certainly hope it doesn't. That is one of the biggest selling points of a cruise.