Arch-Fiend
Explorer
If you still want to do this consider some way of adding in proficiency bonus instead of just starting with such a high number. This means that goblin can still be a threat to a Fighter in heavy armour (at least at low level - bounded accuracy is still a casualty).
i describe a few ways even low level characters can get around armor, such as half-swording, using weapons that are better at getting into niches, and grappling with these weapons and techniques. which i think does a good thing giving grappling more use in the game. low level armors dont have enough damage resistance that you need to worry about low level monsters not being able to damage the party and visa versa. if low level characters are given high level armor by the dm, then well i hope the dm had a good reason for it. however realisticly a goblin skilled as a first level character shouldent be able to damage a 10th level fighter wearing realistic fullplate armor without rolling a 20, unless he uses a dagger while grappling (gaining a +4 bonus to hit)
now, i like the idea of doing something like pack tactics but instead of advantage granting a +1 to hit, then sort of doing the same thing with flanking gaining a +1 so that a guy in full armor getting ganged up on by a horde of goblins is going to simply not be able to guard the niches in his armor.
something like flanking grants a +1 to 2 characters standing adjacent to an enemy where you can draw a line between the center of their square and the center of its square, then every flanking pair around an enemy increases the bonus by flanking by an additional 1 while also granting the flanking bonus to any other enemies adjacent to the flanked character who just dont happen to have a flanking partner, so 4 flankers is +2 6 flankers is +3, so on and so forth. i think goblins would design their homes around exploiting these advantages by doing stuff like having tight tunnels with murder holes that goblins can surround an intruder on all sides but also from above .