I have always wanted to run a DnD campaign where humanoids could be a threat througout the campaign without suddenly having 8th level guards in the bigger cities, 12th level orcs and so on. I would also like magic items to be quite rare, much the same way magic items in 5e are.
Well, player characters are humanoids, and they become 'high level' and powerful, so clearly humanoids can do that. Uber-humanoids just need to be placed about the campaign world in a reasonable fashion. So, no, town guards don't all gain levels with the PCs, but PC's might go from frequenting small towns where the guards are pretty bumpkinish, to big cities where they're fairly competent, to extraplanar outpost where they kick Vrok tail on a regular basis.
Another trick that keeps monsters 'useable' longer is to re-cast a monster with a different role but higher or lower level. So a monster that's a Solo at first level might be re-done as an elite 4 levels higher or a standard 10 levels higher. High in the next tier, the same exact same creature might be a minion. The monsters not changing, just the mechanics used to represent the challenge it represents to the characters.
What I would have to do is basically rewrite the to-hit, defenses and skills of all the monters I would want to use. Something I could probably get from creating a small generic chart for each monster role.
This is really easy. It's a simple matter of deciding which scaling bonuses you want to take away from PCs, and adjusting the 'monster math' apropriately. If you want to take away the 1/2-level, but leave PCs enhancement bonuses, stat bumps, feats, and so forth, just subtract half-level from monster attack bonuses and defenses. If you want to go for 5e-style 'bounded accuracy,' just do away with scaling bonuses for both PCs & monsters, leaving the odd arbitrary '+1' for PCs to pick up and get really excited about.
The alternative as it looks now is to run a 5e campaign, but I do feel that the current draft is just a bit too rough. The upside is that 4e combat has a tendency to drag on a bit and 5e combat looks to be really fast.
As a player, what would you see as the downside to the math flattening?
The main problem is that you lose a lot of the sense of advancement, which is one of the 'hooks' that makes D&D so addictive.