It's a trap!
I've played all the systems you've mentioned (except HARP, when did ICE do something new? Glee!) but conversion between them is extremely difficult the classes are hard to map onto each other as because none of them (except MERP) have a core standard world system - and it's not like the mapped classes between Rolemaster and Pathfinder (for just one example) are that similar anyway.
To add to your woes there are other factors like economics, they all have base coinage systems (which is helpful) but system is the core of your troubles...
- There are two (common) versions of Rolemaster (RMC, RMSS, let's not get into companions and expansions here!)
- There are at least three "common" popular flavours of AD&D (3.5, Pathfinder, 4th)
- MERP looks initially like Rolemaster, but you'll need stats for that too.
- HARP needs stats too!
So even if you choose just one of each of the systems that's four different stat blocks PER NPC. If you want to cater to every possibility that's SEVEN. That's an insane amount of work for the highly detailed NPCs and certainly a non-trivial amount of work for any of the throw-away NPCs.
So what's the alternative?
Generic descriptions and possibly an appendix guide for token conversion for systems.
By Generic descriptions I mean like the descriptions in the excellent Masks book, whereby you are given flavour text, description and capacities of the NPCs that you can map to whatever system you want with a little bit of work. That way the world system becomes available to any system you want (HARP, RM, Warhammer, Runequest, etc etc etc) without you having to worry about stat-grinding every single NPC in your book - which is frankly a waste of your time when what is important is the NPCs motivations, goals and interests.
An example block from Masks includes the following infomation (taken from their free example PDF)
- Appearance: A description of the NPC
- Roleplaying: Guidelines on how to play the NPC, mannerisms etc.
- Personality: Note that it's different to Roleplaying!
- Motivation: Internal goals and motivations.
- Background: A brief history
- Traits: Items like "Crafter, focal, genius" these give you guides to the skill/combat blocks for the NPC
Most NPCs in a big campaign your players won't be fighting; they'll be talking, making deals, negotiating. It's a BIG world and they are very unlikely to be killing everyone! I've used Masks (and this generic method for NPCs) for my worlds for some time now for running both Rolemaster and Pathfinder games and it's saved me a lot of hassle and time.
Prices/Plants/Animals and so on
Which "just" leaves all the economics and monsters and so on. Difficult? No! Just use comparitive economics and again, traits to indicate what you want. Any GM worth their salt can decide (for example) that a "huge bear" in AD&D equals a dire bear.
Cities and dwellings can be described just like characters; except the traits would be something like:
- Appearance : The general appearance of the place, common buildings, stand out features
- Roleplaying : General populace traits, common topics, jobs and economics
- Motivation : What are the things that worry and drive the populace, rival states, nearby enemies, dangerous local volcanos
- Economics : Relative to the standard pricing, how expensive is the place? How easy is it to get key items (see traits)
- Background : A history of the location
- Traits : Fishing, Border town, Capital - traits can highlight the key points of important for a city just like a person.
As to magical herbs/potions and so on. Just describe once again the relative level of access. You can do this regionally; if you want specific herbs or spells then describe them in generic terms; thus:
- Appearance : A wilted black leaf about the size of a human hand with red veins running through it.
- Effects : This powerful combat herb is used by dark elf assassins on only the most important missions, it's been known to kill those who take it too often
- Location : This is grown only in a few dedicated shrines in the blood of victims sacrificed to Naig, the dark elf god of murder.
- Traits : Very rare, Strength, Speed, Addictive, Dangerous
Monsters should be handled just like an NPC, after all, that's what they are really!