While that post title can (and you'd think would, given the content of this blog in general) be applied to setting elements quite easily and quite well, I actually am referring to game mechanics in this case. Never be afraid to borrow a great mechanic, even if it's from a game that's very different mechanically from the one you're running. If it fits in with the campaign you want to run, and will add significantly to the flavor and fun of it, then by all means, let's grab it and go.
DARK•HERITAGE has a number of rules options for the "base" rules, but I also have a number of house rules imported from other systems and other games. In some cases, they come from closely related games; games built off the same d20 "engine" if you will. But in other cases, they don't. To whit--
Although I've got a Pathfinder and a D&D 3.5 option, my favorite and preferred system for DARK•HERITAGE is actually d20 Past, which is a campaign supplement for d20 Modern. In addition to that, d20 Past gives you three setting models, each with slightly tweaked mechanical implications to better suit that setting. DARK•HERITAGE uses the "Shadow Stalkers" campaign model. I've also cherry-picked a number of elements (especially Advanced Classes) from Urban Arcana just to round it out and make the mechanics a bit more robust and create a broader buffet of options for my players.
That sounds simple enough, but right away, I'm stuck with a few needs. Shadow Stalkers assumes a setting that is literally our earth--during a pulpy or swashbuckling era--combined with Bram Stoker's Transylvania. It's the setting of Stephen Sommers' Van Helsing, basically, or possibly Michael Bassett's Solomon Kane, which in an unrelated aside, I'll point out that it's extremely disappointing that there still hasn't been a North American release date for that movie. If you can get your hands on a British (or Spanish or French) import of the DVD and have the capability of watching import DVDs either on your PC or with a multiregion DVD player, I highly recommend it. Pretty good, fun, sword & sorcery fun, with a lot of the stuff I'd like DARK•HERITAGE to have, honestly. Not saying it's perfect, but it's a decent movie. Quite a bit better than Van Helsing, actually.
DARK•HERITAGE, on the other hand, is clearly not our earth, even in a romanticised or pulpish iteration of it. It's a fantasy setting that resembles, in many ways, a D&D setting as much as the real world. So, I had to use some D&D races, some modified, some created whole-cloth. I decided to keep the D&D equipment list and regular spending of money rather than use the Wealth system. I also dropped the Reputation system, since I have no interest in it. And I made a few other tweaks, including changing the rate of character advancement, and making some changes to the supernatural classes to account for the changes in character advancement and still allow them to be viable. A handful of other minor tweaks for flavor and taste.
But D&D (3.5) and d20 Modern were already very similar systems. What have I borrowed from other games entirely? Three things--two of them from D&D 4e (kinda) and one of them from Call of Cthulhu. From 4e, I borrowed the concept of healing surges. This is especially important in DARK•HERITAGE where magical healing is pretty much non-existant. I thought about using another varient for hit points, like maybe Wound and Vitality points or something, but decided that creating Healing Surges, using the already existing Action Point mechanics to deliver them, was good enough.
The third mechanic, borrowed from Call of Cthulhu is the Sanity mechanic. In earlier iterations, I literally used the Cthulhu sanity mechanic, which is now open content and "officially" part of the d20 system (even though it's also clearly a non-native system, being based on BRP's percentile roll based system. I've since used an adaptation that came from The d20 Freeport Companion where Rob Schwalb wrote up a "native" d20 adaptation of the concept, with very similar rules, but which felt like they belonged with the system. This is hugely important to me in reinforcing the atmosphere and "feel" of DARK•HERITAGE, and making sure that it feels radically different from D&D--even though it mostly uses very similar rules.
In fact, that's the purpose of all three of these rules changes--they significantly impact the way the game "feels" in play, which makes the experience considerably distinct from playing D&D, or even "standard" d20 Modern.