With reversion to the old combat formula (or some version probably not far from it) close at hand, and legacy mode pending for some point in the future, it's still not quite clear how exactly Jagex plans to work it all out. Since the Evolution of Combat there have been various tweaks to try to smooth things out, patch this, or nerf that. Setting aside concern about the presence of abilities and their use, one persistent issue has been the relative strength of one combat style with respect to another. In noob's terms, one combat style does more damage per second than another.
Does this necessarily have to be a bad thing? The irony is that with everything perfectly balanced, I think (at least personally) that things get a little boring. Just imagine all three combat styles hitting exactly the same, at given levels. There are full sets of equipment spaced every ten levels or so, all virtually the same except for class designation - which simply indicates whether it's melee, ranged, or magic armor. Fighting with one class against another results in damage by some fixed multiplier that determines just how much of a (dis)-advantage one side of the combat triangle is at. Over large numbers of trials, the side of the combat triangle with the advantage wins as it is supposed to. It's hypothetically perfectly fair, but it's also something else: extremely boring.
What's the alternative? Quite simply, more varied combat techniques that are unique to each combat style. As an example, consider the following setup: warriors could hit relatively higher compared to the other two combat styles, mages could have the ability to stun/bind players, perhaps involving a health sacrifice, and rangers have access to damage over time abilities. Now how would this play out?
Magic vs Melee: Although a warrior can hit higher, a mage can bind them in place, enough to give a critical head start and win the fight, if executed correctly. Magic beats melee, check.
Magic vs Ranged: Again a mage can bind a target, but thanks to an archer's range with a bow and arrow, it won't really help, negating that advantage. With strong damage over time abilities, a ranger should make quick work of a mage, so range beats melee, check.
Melee vs Ranged: Without the ability to stun, unlike a mage, a ranger might get off a quick shot or two, but the higher hits of a warrior will pummel any ranger and handily knock them off. Melee beats ranged, check.
This is just a quick hypothetical example of something that might work, but we do know what does not work. Before the Evolution of Combat, magic was sufficiently expensive to put it out of reach for most players. Coupled with the fact that (particularly f2p) mages had no real defense against the slashing blades of warriors, they never really stood a chance. Of course, melee did naturally have an advantage against Ranged as well. This lead to everyone wearing melee armor, as it was not so much fun to try one of the other styles and become a living punching bag.
RuneScape has the characteristic of not defining your character right off the bat. You can train melee, ranged, and magic as high as you want, independent of each other, and then freely switch between without losing anything (though pures might feel free to disagree). This is one of the things that makes RuneScape great and I don't suggest changing it at all. Rather, since we do have such freedom to try everything out, we're not really excluding anyone by making certain elements unique to one combat style. So why not try it out, and shake things up a little?