I like the idea of difficult PvE.
However, as many others here pointed out, the henchmen AI needs to be buffed as well, to compensate. In no way a rise in difficulty should lead to have areas playable by full human parties only. Having ALL PvE content accessible even to parties largely made up of henchmen needs to be an absolute requirement for every possible change.
Sometimes even today, that requirement is not really fulfilled. There are areas where henchies just aren't up to the job. Take Mineral Springs for example. I can't even begin to imagine what this area would be like if you'd add Priests of Sorrows to the group of 20 Ice Imps in front of the Cave...
By the way, Mineral Springs is a very good example of why the OP's idea is a good idea in general. Yes, Mineral Springs is somewhat hard, but it's also utterly unfun to play. There are only two different type of groups in that area: Avicara ranger spike teams (which are totally over the top by the way, the only way to deal with them seems to be a VERY competent prot monk) and one type of balanced group Avicara. Bascially, there are only Avicara in that huge area. And yes, Avicara get old after you have killed like 200 of them in a row. Oh, and of course they always spawn in the same spot, too. It's one of the most boring areas in the game, despite it's "difficult". Diversity should the keyword here. In Mineral Springs, monsters kill by mass assault and by being overpowered, not by imagination and an intelligent AI like it should be.
Personally I'd not touch existing content that much, but adding new, high level PvE areas in the way SF was made. SF is a very nice example of what high level PvE should be like. SF is a good piece of work because
- there are different types of groups which require/encourage different approaches to handle
- there are enough types of different enemies to keep it interesting
- even usually discriminated against classes like Mesmers can feel appreciated there (Priest of Sorrows and their much beloved Ressurection spell...)
- it's reasonably difficult without being too hardcore
- it still can be played with henchies (with the exception of Orozar and High Priest quests which I don't like for that very reason).
If we'd add some more randomness to SF (in the way the OP suggested), we'd really have a next to perfect PvE area. This is the way PvE should be.