I am looking for sandbox-y random encounter tables, ala D&D 2e, where you had "Hills," "Mountains," "Jungle," etc. In other words, Challenge Rating should not be considered, but rarity of creature should be. I believe previously this was a d100 roll. Where can I find some, in the rulebooks or elsewhere?
[RPG] the random encounter tables?
dnd-5eencounter-tablesmonsters
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Bell Curve Rolls will add some predictability to the game and favor player choices over random rolls. I would just use the tables from the variant to determine any changes, and I would suggest just rolling 3d6 twice for Advantage/Disadvantage to keep the mechanic the same, although as SevenSidedDie points out, the probabilities are similar. Ace in the Hole's equivalent would be 16-18, though since it's a 20th level ability and 18 equates to "automatic success" in the Bell Curve Rolls, 18 is probably appropriate. Otherwise, I think your changes make sense. In the end, equally applied to NPC and PC alike should go a long way in making this variant work.
Check out this question: How can I play monsters and NPCs up to their potential?
The challange and XP ratings assume (I assume) that monsters play to their strengths. For example, your Hobgoblin captain should be commanding the hobgoblins - these are not mindless drones who attack without a plan - these are genetically professional soldiers. Good soldiers do not fight fair - level playing fields are for sporting fixtures, not battles.
The hobgoblins should refuse combat unless and until they have the advantage or they are desperate; but these guys plan ahead so they don't get desperate.
If the hobgoblins are aware of the party's approach then they should be preparing a series of ambushes, fall-back positions, escape/disengagement plans, booby traps, ways of separating the party and destroying them in detail.
If they are unaware then they should have done this anyway - good soldiers do.
For example, an attack on the party during a long rest has possibilities. 7 arrows with surprise at the PC on watch should render them unconscious with a high probability of success. They can then move in and slaughter the remaining PCs. Played this way this is certainly a 2x Deadly encounter. If they survive (which is unlikely) they will have earned every XP several times over.
This assumes that the party really wants "near death battles that bring them out bloody and bruised". My guess is that this is not what they really want.
The suggestion of a "battle arena" test is a good one. This can also be done in the campaign - maybe there are gladiatorial contests in your campaign that are not "to the death". Playing a team knock-out (that ends with 1 team knocked-out) could teach both you and your players what "deadly" can really mean without having a TPK.
Best Answer
There is a section on creating these on p.85 of the DMG which provides as a demonstration of how to do this a table for a Sylvan Forest. There are no other officially published tables that I am aware of.