I would recommend a few things instead of this...
1) Hazlan, Har'akir, and possibly Ghastria are focused on in the next few updates... these locations would benefit from official factions, banks, complete crafting places, quests/bounties, rentals (this would honestly be the most effective thing as it encourages player driven RP provided these rentals are used responsibly. I would recommend for Hazlan that only Mulan can own rentals.), dye dummies with all the dyes as opposed to just like 5 or 10 colors, and maybe a few more areas for RP.
2) Greater potential consequences be placed upon Good/Neutral aligned 'team good' type characters. As it stands there is 0 hard risks when entering player driven conflicts while antagonist characters can easily be put at the whims of local law enforcement. Villainy has everything to lose and can do so at the drop of a hat as it is. It goes against the lore first and foremost as Ravenloft is a domain that generally caters towards and encourages evil play. The Dark powers seek to develop dark hearts and corrupt good ones. This should be reflected IG imho. It is really crummy to have your bad guy meant to terrorize low levels or engage in skirmishes be utterly crushed by level 16 druids and wizards who will either just pin you down with magic or just time stop and end any threat before it ever happens. Is it horror if literal Superman is there to protect you effortlessly?
3) East Barovia dungeons are boosted drastically to help further cement this divide just a little more.
That's just my ideas. The 'curse' is a pretty neat idea but sounds like a pain to implement. It also does not directly solve the issues around higher levels farming low level dungeons or higher levels engaging in conflicts where they will out crush the monster or villain the other player has. Something that would directly solve the issue (My first idea would push characters towards other part of the game so they don't feel like they must choose between Barovia or Port, my second idea is to give Evil alignment for low levels a greater sense of security while they work out their plans afterall; villains drive story, and my third just to sort of keep distinctions clear. Additionally, West Barovia should be a viable RP option as well.
Generally speaking, I agree with this sentiment. It's probably no surprise to anyone that the most fun I had was playing a lower level during a time when many high level MPCs and villains were around, outnumbering and outgunning the heroes. This created strife within the heroic factions and led to interesting RP worth coming back to for months. There was real friction. Every time the sun went down it felt like something crazy was going to happen. The characters involved would change sides in moments of desperation or just give up, frequent skirmishes meant people had a chance to show their stuff, it was great. Having played a (reluctant) hero for most of this time I can safely say it always felt better to not be on top of the world.
The only time I felt that way after reaching a higher level was when there was a DM involved, or players on both sides of the conflict were willing to put RP first and focus on the drama of the story, rather than just kill each other. I became something of an alt hopper to try and enjoy that low level experience again in bite sized portions, but then Port was the place to be and I just didn't want to go most of the time because it doesn't speak to me like Barovia does. We also have a timer now for MPCs that we did not have back then, so I think the average level of AMPCs is much lower, and there are now so many players that reaching a higher level is generally much smoother.
I suppose that most of this stuff being a lot of overhead is a recurring theme; this feature is unfeasible due to potential bugs or performance problems, and really consistently getting Vallaki to feel right for everyone means an insane amount of DM effort. Their efforts can even be increased inadvertently when they need to find a way to make things fair for both high and low levels.
This is kind of a rambly reply but before I get it too long, tldr; I agree, the continued development of other areas gives villains new homes and this has already proven to have some positive results on their development. Part of this suggestion is, at the heart, that other places in the module are not necessarily that attractive to stay around, and I would like for Barovia to be a place people choose not just because it's the only place or the first place but because it's
the place they want to be, rather than where all roads lead, if that makes sense. While I was inactive for a long time I did eventually learn that faction bases were revamped or added and it was really cool to see that. Continuing in that direction would definitely be an A+ move from the devs, it's well within their power to lay the groundwork for new places besides just Vallaki that people want to RP in.