Here is my solution to easy to learn but everlasting gameplay:
You start the game one, maybe two skills waiting to grow into trees. A fighting one and a preparatory one. What they are depends on your class. As you grow, you climb your skill tree and your one or two skills branch out a little and get a lot more badass, so that by the time you reach the end of your first two trees you are as badass as they come, but only in a single style of fighting. So very early on a relative noob can prepare himself for his chosen fighting style, enter battle and use a skill as powerful as any available to me, even though I've been around for years.
The difference between me and this noob is that I have more options available to me, more styles I can use. So: Noob can run around slaughtering your basic mobs but only with his one style, IE whatever few moves have grown from the tree he began with. That keeps it simple for him, but if he takes on more distant zones or a powerful mobs they might be more unique and less susceptible to his style, or have a wider range of styles it attacks with, so his one style--badass though it is against standard mobs close to home--just will not be flexible enough to cope. From there, he gains more experience and invests it in new skill trees associated with other classes. Maybe he actually remorts into that class, maybe not, I don't know.
In this way he gradually expands the options available to him and the complexity of his gameplay can increase, without any extra raw power being available to him.
How to make the game last for oldbies? Make the cost of investing in these skills/spells increase exponentially, so that your first couple of trees are cheap and easy and you rip through them at a glorious rate, but by the end it's effectively impossible to master all possible styles. So you can keep killing etc forever, and you will always be gradually expanding your character and collecting more options to incorporate into your style and becoming less and less defined by any one class, but again, no more raw power will be available to you than you had all those years ago.
This would require a massive amount of work because it implies completely revamping skills/spells, finding a way to make each tree powerful and unique in its own right, and balancing them such that just throwing on the preparatory skills/spells available to you doesn't do you any good--only carefully selecting the relevant ones does. Also classes, levelling and pretty much our entire gameplay system would probably need a complete makeover.
I think that should be the direction we move in though. Customizable races, augments, and elements are all designed to gradually open up options and character flexibility in line with this, and a complete system would factor in all of them.
To me, that's the direction we move in. It does require thought and planning, which is what threads like this are for. On the other hand, I've been through massive code changes before and I've helped to test a lot of new features, and in the end you simply cannot get everything right on paper. No matter what it is and how much or how little has changed since it all began, such things take endless testing and tweaking ANYWAY before they can even get close to where they ultimately need to be. Claiming that changing this or rebalancing that might throw off something not even being tested yet is ridiculous.
Fortunately we have a small, understanding playerbase who have hung around even with a code as shit as ours is. We can't exactly go backwards.
This post has taken longer than expected.