Work on the class balance. It's been 8 years of CoA and even more of RoM (am I wrong?) and most of the combos are still not welcome in end game instances (other than as leechers I mean). There are 2 ways to balance the things - either try making them somewhat equally good or try making them somewhat equally bad. Even Hackman can see that the only correct approach is the former, the latter will make the affected players "love" you.
Meanwhile, a good compensation/stimulation might be an increased number of items in the loot based on the number of the non-mainstream/non-meta/non-imba/non-OP combos currently in the party. You should be able to gather (with easy) the info which the most frequently used combos in instances are and of course which the least frequently used ones are.
Consider asking the players to assist you with the balancing (for free). There are 110 combos, each one of them has pros and cons (mostly cons :D) and I Doubt you, devs, have the judgement, the knowledge and the XP with all of them - don't try making combos-affecting decisions on your own (again) - we are well aware, devs, that it's impossible to actively write code and invest enough play time to test its consequences at the same time.
//===================== Edit =====================//
Please do NOT add 80, 90 & 100 level elite skills contrary to what a few people suggested. For those of you that aren't good at math, adding these elite skills means introducing 3 * 110 = 330 new skills! That's massive amount of work, tons of bugs (as if there aren't enough already) and most likely new imbalance issues. Also, this will decrease the chance for introducing a new class drastically because it will increase the number of the elite skills needed to be introduced for all of its combos.