Should blackrock infection sweep the world?
A few assorted thoughts on this question:
There's more than just character-power at play here, there's player experience. Over the years, we've learned how to play the game - even if everything was rescaled numbers-wise to the same difficulty, we still wouldn't have that same initial experience of jousting ettins because we as players know better now.
Simultaneously, new players (when we have them) are just as green when they enter the world. Yes, the various expansions have been an across-the-board-buff to players, but people are still pretty clueless. I started (aside from a one-month-trial) playing after AoS and it was still a white-knuckle experience to be jousting a lone ettin in my first month. If people don't get accelerated by joining a guild, the low end of the game is still challenging for a long long time (although ML has pumped up the power of starting characters).
There are still many areas of the game I don't dare to tread in because I know if I die there, it's a long, long trek to get healed and my odds of getting back to my corpse are not great (this includes most of the ML dungeons and even parts of the classic dungeons except on very specialized characters). Buffing the monsters would mean that there are going to more areas I'll be scared of. (this isn't automatically bad, just that I play the game to my confort level, not to where I have defeated every possible challenge)
Scaling loot is hard - because making monsters more difficult will mean the value of already-accumulated wealth will be much higher. IDOCs will be even more valuable. It's also no fun when you're neighbourhood goes to pieces and you discover its too dangerous to visit your own house (I feel a deep, deep sympathy for the people who had houses on Magincia beach).
As much as I like just naturally allowing new, harder, more rewarding lands to be added so that everyone can rise to their natural level of challenge at their own pace, it leaves the ratio of "new player lands" to "veteran player lands" is out of whack which seems a little wasteful and forces people to leave behind areas they are attached to.
The "bright paths" concept (of which I assume New Haven was just the first stage?) is a nice way to keep players focused on challenges appropriate to their stage of development. I think it also allows lands off the bright path more freedom to be made more challenging.
I think monsters deserve expansion packs too. It's interesting to me that the orc and gypsy camp mechanic has never been revisited to create more "dynamic minilairs". I think monsters could use AI personalities (randomly make some orcs berzerkers who run, some aggro against people who speak words, etc) and leaders (you! you! stop running in circles and flank that archer from opposite sides!)
But adding more umph! to monsters isn't enough, even if their loot is scaled up - there is also the problem that there are simply many, many places to get loot. Think of all the orc camps in the land. What would make you want to visit one and not another? If they kept the same graphics but were quietly upgraded to ogres or even slow-spawning troglodytes, would that make them any more appealing to challenge? Or would the novelty quickly wear off?
UO feels like it's overlands spawn needs new systems, not just a power boost.
I'm a fan of Delgor-like triggered encounters (either randomly triggered by a person walking into the area or by some more complex circumstance like casting a spell, or having an unwelcome pet at your side).
Or, using the collector's quest as a guide, have quests where you need to visit 4 random spawn sites throughout the facet you're on, provoking a minibosses to spawn or have treasures to steal and assemble the pieces for a reward/key
Another possibility is to give incentives for people to play "classic" characters. What if there were special rewards or alternative loot which might drop for monsters defeated with only a certain subset of skills/equipment or who travelled only on foot? (we have some of the mechanics for detecting/filtering between the Magincia spawn and Richardo trial). Say, old-fashioned "power/mystic/vanquishing"-labled weapons would drop if you were fighting equipped with only similar gear? Systems like this would allow you to use the same monsters, but have them offer a greater difficulty by encourging players to nerf themselves.