[robertsconley]

I been running my take on Judges Guild's Wilderlands of High Fantasy, the Majestic Wilderlands, for 40 years. Started with AD&D, moved on to Fantasy Hero, then wound using GURPS for two decades. Along the way I ran session and campaigns using other systems like Harnmaster, Fantasy AGE, D&D 3.X, D&D 5e, and my house ruled OD&D take, the Majestic Fantasy RPG.

My opinion from dragging my setting across these system that RPGs have three broad sections

system, advice, and lists.

The system are the mechanics about how to get something done or how something is defined. Attributes, how characters learn things, attempting a task, fighting in combat, monster statistics, etc.

The lists imparts much of the details of the RPG itself. This this can encompass, classes, skill lists, items, spells, monsters, NPC types, etc.

Advice in encompass material about how to run a campaign, play characters, create adventures along with aids like encounter table, generation tables, monster difficulty guidelines.

I have found that by altering the list and advice, one can to a great (but not inifinite) extent repurpose a given RPG for a given setting. The key is understanding what the system and what are the list.

For example in D&D 5e, using the three book the 5e system feel like how many edition D&D felt like over the years. Mishmash of various fantasy tropes mixed together into something recognized as D&D style fantasy.

Take the same system, altered the list of classes, skills, monsters, items, and repurpose feats as virtures along with some good advice with aides, you have a Middle Earth Roleplaying Game. Same system, completely different feel.

This is something I have found personally to be true when converting my GURPS Majestic Wilderlands material to use a modified version of Swords & Wizardry a classic edition clone.

The goal is not 100% fidelity but to be close enough that the original setting material works as is. While being able to use classic edition material from other sources.