A common opinion I see about Dungeons 21-25 as seen in Plarium's video on future plans is that it feels like a lazy change... a cop out in a ways.  They are still going to essentially be the same boss but with bigger numbers and the same 5*-6* rewards and no assurance of significantly better odds on 6* or rarity than the current level 20 dungeons.

So my own approach would have been to instead introduce Set Spotlight versions of each of the dungeons.  Affinity of the dungeons will change every three days in a 12 day rotation.  On each day, 3 versions of the spotlight dungeons will be available, each with a different spotlighted set available, getting switched to the next batch of sets each day.  When running the dungeon itself, the game will treat it as though each enemy has 6 pieces of that gear on them (A Lifesteal Dragon Dungeon will allow all enemies to heal for 30% of damage dealt while a Speed Dragon Dungeon will grant all the enemies 36% more speed), including the Boss.  Upon completion of the dungeon, the rewarded gear will put additional weight towards the set the dungeon spotlighted, as well as guaranteed reward of materials for constructing gear of that set in the Forge, ensuring even the unluckiest players will be able to get gear of their desired set at a reasonable rate for the resources they spend on this feature.

Spider would instead have each affinity last for as long as the FW rotation before switching to the next, giving you the option to instead spend your FW keys along with energy on the FW Spider for a guaranteed accessory drop for that faction instead of going for the Glyphs in Faction Wars.  However, like Faction Wars, you will only be able to field a team of the relevant faction faction.  The reward is again weighted in favor of the chosen faction, but also drops materials for making accessories in the forge for the chosen faction, but I feel it should come in two difficulties just in case a "Spider20" equivalent Faction Spider might be more difficult for some factions than others... or make the spider "Alliance Limited" rather than "Faction Limited" to accomadate for this difficulty difference and make the forge material that gets dropped usable for constructing accessories for any of the 4 factions that are accommodated by that version of the Faction Spider.  I feel this particular idea still needs tinkering however.