I've got a few ideas about how to make the instances themselves more dynamic and challenging for players across the board. Rotating the middle bosses in AC was a good step towards making the instance repeatable, without becoming overly boring right off the bat.
1. I'm a bit surprised that as of right now, Sardo Castle remains the latest instance to have incorporated actual runes as a method to hinder players' progression. I'd like to see another instance designed in which the runes themselves are crucial toward advancing through it. Bosses should be frequently tossing out different attacks based off of runes they used, with players given a chance to stop the aoe/attack by using a counter rune. A final boss made of nothing but runes and runic energy would be pretty cool to see. In addition, utilizing more than seven different possible rune combinations and effects per hallway/boss would go a long way towards making the instance more difficult to clear.
2. Loot! It's like the bosses only ever think about gear and weaponry being important. They don't ever hoard things like scrolls, books, or any of the other things a player may consider to be valuable. A boss with a chance to drop an x rune, driller, puri or plussing gem would keep players coming back for more. (Drop rates on those items would work similarly to the way the gear and weapon drops do- i.e. the higher the rarity or level, the less chance of a drop).
3. Player designed dungeons. Give the community the opportunity to design their own dungeon layout within a given set of parameters, and then put it into the game. Factors to consider are the aesthetic appearance, obstacles and mobs (different kinds of attacks, placement, types, and how hard they hit per instance level, etc.), bosses (loot drops, attacks, defense, hp, dodge rate, resistances and enraged state), and layout (how many hallways, staircases, impassable sections (a maze with changing paths would be kind of awesome), number of boss rooms and physical location of the instance (i.e. where is it within the existing map, or does a new zone need to be created for it?).
4. Shrines to the gods and evil things roaming around Taborea. When clicked on, these have a chance to provide a buff or debuff that can help or harm a player progress through a dungeon. (These can take into consideration which class is clicking on the shrine, in addition to which god/goddess/dragon/other entity it is designed for.) They make the instances a little more interesting if the player is finding it too difficult or too easy to clear.
If you happen to have any of your own ideas, please post them here.