1. Instead of adding a savage ultimate 8 man raid, they could add savage and ultimate 4 man dungeons that no, you cannot queue these on duty finder.
The Great Hunt EX. It's not a dungeon, but it is still 4-man. The only thing really missing are the large pulls of trash before getting to him.

2. Adding a monster that needs to be killed immediately or will cause a party wipe and this will prevent tanks from pulling way too many mobs and wiping the whole party.
So you mean bosses?

3. Have monsters in the game that will do nasty debuffs if they aren't silenced prevented which will make it even harder to complete. No one wants a dot tick for the whole dungeon on the tank which makes them take more damage. The healers would love it as they don't have much else to do except damage and healing.
1.0 dungeons like Aurum Vale would place a DoT on everyone just for being inside the dungeon. Wanderers Palace HM has a dungeon mechanic where valves need to be turned off, and there is another 'toxic environment' dungeon that slips my mind. These kind of things are not engaging or fun; they are just annoyances meant to slow down the run.

4. Adding rare monsters that spawn in dungeons like mini-bosses. There is no guarantee that they will spawn but if they do, they can drop mounts or really good glamours.
This is the best idea you have right here. This will get me inside a dungeon.

5. Making bosses in dungeons 4-8 men have abilities that can be stunned and if you don't you are dead. This means the whole party has a responsibility to stop it happening. This doesn't mean that all abilities they cast are stunnable, just this ability right here and it is important for the tanks or the people fighting the boss that this ability is about to go off and you should have time to stop it.
Again, there are ARR dungeons that do exactly this. There will be objects players have to interact with in order to stop the boss from going HAM. Halatali HM has it, so does Hulbreakers Isle HM, among others. Even today's dungeons still somewhat have this such as the curtain boss in Cosmo Canyon. Interact with object or die.

6. Finally removing diminishing returns so everyone in the party has their own diminishing returns instead of it stacking up.
Not sure what you mean here.

To close, dungeons are hackneyed content that the devs have been unable to keep interesting beyond their initial run. As PvE content, I don't want to see them go away, but tears drop from my eyes when the devs spend time on max level dungeons when most players at max level don't want to be inside dungeons. That's where they just spent all their time leveling.

EXP gain is probably the greatest incentive to run dungeons. Once that is no longer needed, even the best designed max level dungeon in the game is not going to get much praise when players run it for the umpteenth time slogging through it for tomes.