balancing solo dungeons for different classes

On WowInsider, I saw this comment and similar like it:

Sounds like a great idea in theory, but just trying to balance a "solo" dungeon for any character class seems to be a bit daunting. How do you balance it so a squishy mage can crowd control and burn mobs while a tankadin can't CC and needs to tank try to DPS?

Good question. My answer would be to design dungeons such that they are not so damn linear. Provide a multitude of alternative paths from the entrance to each of the bosses - in Shattered Halls you can skip some tedious oozeling trash if you have a rogue with lockpicking, or an engineer with a seaforium charge; in Karazhan there's a seldom used route from the stables to the back entrance, another route that starts from Opera and goes upstairs, and there's also the kitchen off the side of the Moroes encounter .. I have no idea where that goes.

In Kara, these alternate routes are a failed design - everyone ignores them simply because it's just more trash to clear and there's no advantage. The design of the locked door in Shattered Halls works. Apart from that one encounter though I'm struggling to think of any others, which is a real shame as this is a missed opportunity.

Imagine some dungeon where behind door A is a series of trash requiring lots of AoE, while the trash behind door B is handily despatched via CC, while door C is locked and leads to encounters which can be avoided via stealth. Someplace further in there's a long winding path downwards requiring either lots of tanking or clever ranged attacks .. or some classes could skip all that by jumping off the edge and slow falling, levitating, or bubbling.

We've already seen that some existing dungeons are more suited for certain class combinations than others, some dungeons go real easy if you have a warlock, others require plenty of CC, and so forth. Just put all that into the one dungeon, with alternative routes.

Update: another timely post at wowinsider reminds me of some old world dungeons with alternative pathings: Dire Maul, Blackrock Depths