For easy 'stereotyping', I'd like to see goblins that are goblins. No 1st level goblin and 20th level epic goblins. If a goblin is more powerful, there are visual cues (plate armor, musculature, unusal size, goblin chief surrounded by guards, etc.) If monsters are classifiable that way, monsters by tier can be threat-classified simply by reputation.
I actually think 4E D&D has an additional, excellent facility for dealing with this "power divide" - albeit a somewhat "anti-process-simulation" one.
If I take a 10th level 'standard' monster in 4E, I can through quite a simple transformation represent it as a 1st level Solo monster, with similar powers but with different attributes (defences, attack bonuses - more attacks but with lower damage, and so on) - and worth the same amount of experience if overcome. I can do exactly the same to "recreate" the same creature as a 6th level Elite monster - or as an 18th level Minion.
I am not suggesting, here, that these are, in the game world,
different monsters - they are the the same creature(s), but represented to interact with different characters. The result is a more interesting and fun experience at each encounter; a party of 5th level characters might be capable of overcoming a 10th level monster, but fighting a 6th level elite is likely to be a much less frustrating experience.
That is not to say it will be any
easier - but attacks will hit more frequently (doing proportionately less damage) and damage done to the characters will be somewhat more controllable - will arrive in smaller aliquots. Furthermore, the hit chances will fall into the range where tactical ruses to gain or prevent advantage will have some real effect on the outcome, rather than changing a "hit on a natural 20" to a "hit only on a 20, but it's a critical if you hit", or changing an "almost certain to be hit" into a "still almost certain to be hit - but maybe a tiny bit less likely than before".
This is not "full scope simulationism", by any means, but it is at least a useful mechanism to permit some "mingling" of power levels, provided you are prepared to treat the rules systems as a tool for adjudicating the effects of the characters' interactions with the world, rather than as a model for the "physics" of the world itself.
It's a hard old trope to let go of, but it offers considerable advantages to do so.