Back in early 2012, when I was gearing up for an Openquest campaign set in the Dawnlands, I came up with a bunch of house rules for Openquest. I thought it would be useful to go through them and pick which ones I wanted to keep. I thought I'd do a bit of retrospective on these, having playtested them and run a successful campaign using them.
My weapon and armour creation system for Openquest, with the rules for calculating ENC. In hindsight, I should use these to pregenerate a weapon and armour list rather than hoping to do it on the fly. It also needs something to determine when a weapon gets the flex, set or range qualities, and I need to rewrite the set weapon rules so they're more relevant and useful. This ruleset has a tendency to generate swords as dealing 1d6 damage, rather than the typical 1d8. Specifically, "longer than a metre" should become "longer than 60cm" (two feet, in moon units).
My teamwork rules. These work well. My experience was that the most confusing part was collaboration, where PCs are rolling different skills. The difficulty wasn't mechanical, it came from trying to explain how their alternate skill was relevant. There were just a number of times the PCs wanted to do it and couldn't figure out how to have it make sense in the world. I don't think this is a problem with the rules though, the PCs just needed to get into the right head-space.
My new major wound table worked well. I learnt that one piece of information I should keep at hand about PCs (along with their Evade, Persistence and Resilience scores) was their major wound threshold.
My overland travel tracker is straightforward, and was the beginning of the line of thought that brought me to my procedure for exploring the wilderness in Swords and Wizardry. It could probably use an update and freshening based on the intervening years of development. For people who don't want the complexity, it's a simple way to determine how far the PCs move in a day, and what they run into.
My mounted combat rules work well and don't really need to be changed. They're a straightforward improvement over the baseline Openquest rules (fewer numbers change, but more options open up). I hate the "Riding is your skill cap when riding" rule in Runequest, since it basically turns Riding into a skill tax.
The movement, called shot and "free hand" rules here all work well. I would keep them unchanged. I did allow characters with a free hand to initiate grapples using them, which occasionally gave things a MMA feel as characters would hack at one another with swords and then suddenly lock up into a grapple and resolve the fight through that.
Competence bands are basically just a procedure for doing what plenty of other referees in Basic Rolepaying games do anyhow. They work and are simple to use in play.
Abolishing attribute differences between species worked fine, though occasionally people wanted some slight distinction between their jackal-headed brutes and their elvish archers. I think what might work well here is to grant each species a single distinguishing trait that is not merely an attribute difference or percentile bonus to skills, but rather allows you to use one skill in a way no one else can - dogmen can bite with unarmed for extra damage, dwarves can use Perception to see when it's totally dark, etc.
Advanced plunder ratings didn't work well at all. The system was too complicated to easily parse, and involved making numerous decisions about what a create did or didn't have that merely added an extra layer of adjudication. The options for modifying this are either treasure generation tables with types, similar to old school D&D, or to simplify it drastically down to the two most important factors - what loot does the monster have, and how valuable is its body as loot? I should write something about this, but I now use a simpler system where every monster is ranked from A to F in terms of its loot, and then has either a + or - for how valuable its body (or body bits) are. +A would be a dragon or demigod, a creature that both has a horde, and is priceless when it's knackered, while a F- is a creature with nothing whose body is near-worthless. If you want to encode a bit more information in the notation, you can shift the + or - to either side based on whether the loot is in its lair (on the Left side for Lair) or on its person (the right side) as I did above.
Abolishing spell ranks didn't give me the results I wanted. I had a few PCs with high POWs who were able to pull off extremely high spell ranks very early in their careers, which made them disproportionately powerful in whatever campaign they appeared in. I think the correct solution for what I want is to expand the rules for divine magic (which I ported over from MRQ2 / RQ6) and use the tens digit of your relevant skill to determine the spell's magnitude. The 5 IP for new spells was good though, and I'd stick with it. It gave each PC a few signature spells they used over and over again, which also reduced the amount of player skill and attention required to manage spells, especially buffs.
Abolishing the common magic skill is the one that I still haven't made up my mind about. Some players really struggle with wrapping their heads around using other skills, and some love it. The rules themselves work fine, it's mainly an issue of playstyle. I think I'm going to keep on using this, but I expect it to play differently if I adopt the above-mentioned rule about spell ranks. So you'll use the tens-digit of whatever skill you use, instead of Battle Magic Casting. I also think I'm going to ask PCs to pick 1-3 skills off a small list that are the skills they use to cast magic ahead of time to help them get a clearer idea of how their own personal style of magic works. When I initially playtested these rules, I let PCs pick whatever skill they wanted in any given situation, but this meant a lot of people trying to use "abstract" skills like Perception and Influence and Language (Own) because they saw these as requiring the least amount of preparation and effort compared to Natural Lore or Craft. There was also the occasional attempt to piggyback battle magic spells on other spellcasting skills, like Sorcery or Religion (Own), though I discouraged this whenever it occurred. I think locking PCs down to a handful of prechosen skills will encourage them to more clearly conceive of how their character cast spells, which should avoid most of the problems. If that doesn't work, I'll probably just make Language (True Names) [a Language (Other) skill for everyone) the skill one rolls.
Anyhow, I plan to continue to experiment with new rules and variations, though it's been a bit since I've run an Openquest campaign.