I do agree that a well thought out Random endless dungeon would do a lot to give this game the replayability that everyone wants.
The MAIN thing it would have to have is Randomness
... Every floor should be completely random: Random layout (we don't want to run the same things over and over, we can do that now), Random environments (and plenty of them), Any monsters from any act can spawn on any floor, ect.
The best way to do it would be to have a Free Roaming mode open up after you defeat Inferno Diablo (come on, its really not that hard). Add a quest into the free play mode that makes you go all over all 4 acts looking for stuff to open the dungeon, once you open it, it stays open and you can go back into it whenever you want but it starts over at floor 1 every time you start a new game.
At MP 0, Floor one would start with level 61 monsters and the level would go up every few levels with random Bosses thrown in. The key would be very high monster density with Lots of elite packs.
Adding the ladder for people to compete with how deep they can get would definitely suck in the competitive players. :)
As for your points:
1. Its truly random - at any floor you can randomly encounter treasure goblin, uber bosses, elites, chest, and etc.
This would be good, but I would keep the bosses on specific boss floors, but those floors can be randomly placed so they don't fall in the same order or places each time.
2. You can only go up in floors, but not back down to the previous floor
I don't see the point in this, who cares if someone wants to go back to a floor they were already on? Its ether empty or mostly clear anyway.
3. Dungeon will act as an end game ladder, so every season, people will compete on how high they have gotten into the dungeon
Exactly, lots of people like trying to prove they are the best :)
4. Every time you die in the dungeon, you drop 10 floors
No. This would just frustrate people, everyone wants to Progress, kicking them back would get as annoying as the rez timer is now... and we already know everyone's opinion of that.
5. The Dungeon should be a separate act 5, no MP, and no TP to town
No, Separate yes. But MP should apply if you want it to be harder and there is NO reason to deny access to town. I like going back to town to sell stuff, if that got taken out it would annoy me to the point I wouldn't want to play and the point of this is to give people something fun to do in game after they beat it.
6. There will be various end of the floor hubs for repairs, shops, and sending items you found in the dungeon to your regular stash
You could do this but just leave access to town open, simpler and better... but it would be cool to see special merchants that can only spawn deep in the dungeon that sell GOOD items that people would actually want to buy. 50k pots, random Rares and very rarely Legendary items maybe.
7. The higher you go in the dungeon, the better the MF, so people who is not interested in competing in the ladder are still rewarded to venturing into the dungeon
Very big Yes to this. Maybe just allow NV to stack 5 higher every 20 levels or so.
8. The skill build you pick to enter the dungeon is what you stick with. You can only change the skills at certain rare shrine spawn or at a cost of 5 floors
Again no point and would just frustrate people. If you want to give up your NV stack then you should be able to change skills. People like experimenting, let them.
9. Friends that attained higher floors in the season can freely join friends that is currently at a lower floor. IE. I have gotten to floor 100 and my friend is in floor 60, I can drop in and play with him at floor 60.
Sure, I can see this. I would just make it highest floor you have ever personally gotten to is the highest you can "drop" into but I can also see this as being hard to program in...
10. When I log off, it will save whatever floor I got to, and allow me to resume at that floor on my next session.
Every game should respawn the entire dungeon, but I would add portals into the dungeon every 10 to 25 levels that would allow you to skip ahead once you unlock them. The portals would reset with each season.