No matter how we try to make money off of this, people are going to complain, that's a given. But there are a few pitfalls we can avoid.
Build a robust cosmetic system into the game, and void adding cosmetics that have to be "earned" or "unlocked". Most cosmetics should just be available for a fee in the in-game store, nobody ever seems to complain about this. Its also good marketing, one player sees another with something cool, things "i want that!" and boom, impulse buy.
Store-bought cosmetics should be bound-to-account. If we offer cosmetics that can only be obtained in-game, they should always be unbound. Anything like event items should be given away in sufficient quantities as to be readily available. Basically, if a player sees something cool(cosmetically) that they'd like to have, they should be able to get it without a backbreaking amount of effort.
This would be an ad-on "quality of life" item, basically its something to help organize all your clothes. A surprising number of people like to play dressup with their MMO characters. We don't have to milk them for every penny they are worth, but throwing in fun things could help.
Quality of Life Enhancements
This is going to be the big one. Basically, we need to design-in certain road-blocks, then sell bypasses. These don't have to be major, a good example is storage. Players will happily pay for more, let's sell it to them. In-game travel is another good one, but we should NEVER micro-transaction "by the ride". Something like reusable teleport items for real-money, especially if they let you teleport a guest, would be very well-liked.
I like to think DDO's model of splitting things out into individual packs and expansions is a good one, but I don't really know.
Anything at all that could remotely give one player some kind of advantage over other players should be avoided. Especially egregious if these have direct game-play benefits. Indirect, such as experience boosts, may be forgivable.