In the days when the Underworld was called the Lost Lands, I went through a number of possible system options. It was originally designed alongside a homebrew system created in order to fill the requirments of Matt Finch's Quick Primer on Old School Gaming while still being appealing to the new schoolers I played with. It did this with some success, but it lacked the kinds of guidelines I like when making wilderness and dungeon areas (such as No. Appearing and Treasure Class). I toyed with using the "Lost Lands" as the basis for a Swords & Wizardry based game, even going so far as to write up a pretty extensive house rules document that is now lost to the ages. I've even considered using a combination of yellow book BRP and the Bronze Grimoire to power the setting, but nothing came of that either.
Now that I've picked it up again (as a side project) and given it a bit of a face lift, I need to think about what system I'm going to use to power it. Obviously, I could go the route of Scott of Huge Ruined Pile and make all the material as systemless as possible. I don't feel comfortable with that option. The amount of changes that would have to be made to any given system to accommodate just the monsters is pretty large. Almost all of D&D's monster list is completely useless. The only exceptions being certain undead and summoned monsters. I'm fine making a wholly new monster list, but I wouldn't want to burden some other sap who decided this setting was the bomb and wanted to use it in his or her home game. Therefore I need some system I can provide stats for, though I will probably put out little handouts on how to run this setting with other systems.
So what system? I'm currently getting better acquainted with Labyrinth Lord since the group that I will be DMing for is relatively familiar with that system. I'm also a fan of LotFP:WFRPG. So some sort of B/X D&D hack? Well... no. I've never really been comfortable making monsters for those systems as I never know how much xp they should be worth. The most likely answer is the Swords & Wizardry White Box, which is by far my favorite retroclone. It has a bare bones sensibility that I think will lend itself well to tinkering and this project in general.
Of course I could pick up another side project that would fill a similar niche in my creative efforts but not be as arduous a task. Possibly more on that later.
With that out of the way, I need to decide what character types I wish to represent with these rules. Fighters and Magic Users fit in easily to the setting. Clerics stick out like a sore thumb. They fit great in Nightwick Abbey's pseudo-Medieval milieu, but their holy magic and mace wielding antics are a poor fit for a setting where even the nicest gods have terrible serpents for pets. I could just stick to Fighting Men and Magic Users, possibly importing LotFP:WFRPG's encumbrance rules and lack of weapon/armor restrictions, but I'm not sure about that yet. I feel like I want to allow some other class options for the players if only to give them more options.
That being said, none of the other classes developed for TSR D&D strike me as appropriate for this setting. Thieves could work, but I find that they generally muck up OD&D. Still they are an option. The others work less well. I'll probably have to create some of my own with the setting in mind, but I'll burn that bride when I cross it.
To close, here is an inspirational image for the setting:
Now back to thinking about Nightwick Abbey...