An end game area, in my opinion, should offer some challenging game play with high replay value, some decent rewards, but not something that sparks a harsh excluding farming situation (aka tombs, no luck with PUG's if you did not fit into a B/P team).
Today it is a huge difference between a good team experienced in an area and one equally good team going there for the first time. To do Warrens or the Deep without any previous knowledge or test trips is a huge (or impossible?) task today. An experienced farm team can do it routinely in a bit over an hour mainly since they know before hand almost every single group and move/combination necessary to do the mission. So even an incredibly challenging mission is with time reduced to repetative farming runs. All due to the predictability of the monster spawns. Today it is almost as hard for a player to go through tombs/UW/SF for the first time, as it was in the beginning. However, no one see it as an accomplishment due to the massive routine farming of the area. I think that is pretty sad, a end game area should be designed to keep up the challenge over time, the predictability destroys that.
Also today, too long trips (elite missions, FoW/UW clearouts) exclude casual players that are not able to dedicated 4-7 hours on a trip. The challenge aspect have so far been solved by combinations of large amounts of high level monsters, huge areas and "death traps".
Death traps (UW/FoW quests, elite missions) are devastating for unprepared first time parties, but only annoying after that.
Large amounts of monsters are not a challenge in itself. If you have a working build killing 100x 10 monsters are as easy as killing 10x 10 monsters, only much more tedious.
Huge areas might make up for some very nice landscapes, but too large and it just only gets more time consuming and excludes people with limited time.
Suggestion:
First:
Random monsters! Randomise monster stationary spawns, patrols and pop-ups, their group size as well as the monster types. This will hugely increase replay value and probably stop most of the "repetative farming" seen to day. You can of course still farm the areas, but not like today by just follow the same movement patterns with a standard optimised build.
Second:
What if a new endgame area consisted of say 7 smaller areas, surrounding an outpost. You can access each of them separately for 45-75 min of hard challenge with some decent drop chances.
However, if you enter the "no 1" area you are also able to enter the next area when you have cleared it/done some quest. By going through all the 7 areas on one single trip you then get some slightly different content (additional bosses/quests etc) than just taking each of them separately. For each additional area finished on a single trip the rare drop probability could be increased by 15-25% as an additional snack. Completing a full run could also give you a title and maybe some other goodie.
One additional possibility is to gradually increase the difficulty when you take the whole trip, so the last area would be significantely harder when taken in sequence than when taken as a "stand alone", but then it also will give significantelly better drops.
With this solution all areas are accessable also for causal players, but the really dedicated ones will get some additional rewards.
Idea for rewards:
Going through the whole area in one go should give you some special reward. But I would like something that do not start a farming frenzy like in Tombs and SF. If you go through a 5-7 hour trip with very hard random spawns you should get some visible reward, but not something you can sell afterwards.
A title or/and unique armour is one possibility. Another I like better is a customised weapon. After defeating the end quest/boss you get to choose a unique skinned req 7 weapon of any type and get to choose inherent mods and upgrades from those possible in the game. The weapon is then customised for that character. So a warrior can on one trip get a sword, a shield on the next, maybe a bow next etc. Replayable rewards but no overheated market. The market thing will of course be escort services to take people to the end, like the spider capture trips.
I'm eager to see how they have solved this for thew Elona end game area.

Regards,
Cloudbunny