OD&D, AD&D, and/or 4th edition may offer some rules that would compliment 3.5 edition (such as skill challenges from 4th, although most DM's probably implement skill challenges anyway).
I don't know that any other edition has rules I would directly import to any other edition, but you can use ideas from prior editions to inform your design of house rules for each edition.
Honestly though, it is GURPS that provided the most inspiration for changes to my house rules for 3.0.
First, I much prefer 3.0 on the whole to 3.5. I did directly import a few fixes from 3.5 to 3.0, but for the most part I stayed with 3.0 as a better overall implementation than 3.5. So, if anything, if you are focused on 3.5 I'd first encourage you to look at 3.0 and figure out what you wantto use.
Things inspired by other editions:
1) Fouth Editions triple HD at 1st level: One of D&D's long running problems is that 1st level lacks granularity. There isn't that much difference in terms of hit points between a wasp, a rat, and a normal human. This results in the infamous house cat problem (the farm cat is far more dangerous than the farmer than owns him), and a tendency for 1st level characters in D&D to be a bit of glass cannons. I didn't like 4e's exact solution to the problem, but the slaying of the sacred cow did get me to thinking about the problem. Ultimately my solution leveraged a different D&D idea - size class. Instead of increasing the HD of 1st level characters, I gave all creatures bonus hit points according to size class. For example, a fine creature like a wasp recieves no bonus hit points, whereas a medium-sized one recieves an extra eight. This reduces grit slightly, but it solves the house cat problem, makes 1st level characters less of glass cannons (solving the rat in the basement problem), and solves the problem of large herbivores or whales requiring large HD and the associated large BAB and other sundry effects. And, as an added bonus from my perspective, it solves the 'Puss in Boots' problem in that that narrative of puss and boots where the cat tricks the ogre into becoming a fine sized creature now works under the rules.
2) Staying concious below zero hit points: Based on my house rules for GURPS, I changed the way things work when you are below 0 hit points. Specifically, you need only make a save to stay conscious on a round in which you take damage. This allows there to be a state in which you are actually wounded that lies between mortally wounded and well. Similarly, I expanded the range at which you are staggered upward into the positive hit points, so that for example, a character with 40 hit points becomes staggered (but not yet dying) when at 4 hit points or less.
3) Casting in combat is hard: Imported from the feel of 1e AD&D, I got rid of casting defensively and I removed the ability to take a 5' step AWAY from a target without provoking an AoO. Now, you always draw an AoO casting in melee, and if you step away from a target such that the target is no longer in your threat zon, you draw an AoO for doing so (this incidently, neatly explains why wizards use staffs). This helps improve the feel that casting in combat is hard (though I still think there is a ways to go, more on that later).
4) Being surrounded is bad: Imported from the feel of 1e AD&D, I've added several degrees of being surrounded beyond that of 'flanked' - namely 'surrounded' and 'encircled' - that add greater bonuses to the attacker. That's the closest I've come so far to restoring facing.
5) The 'mariner' class: The direct ancestor of my homebrew Explorer class for 3e is the 1e Mariner class from dragon. Like many of my homebrew classes, it is more generic than its ancestor (see Barbarian vs. Homebew Fanatic, Druid vs. Homebrew Shaman, Paladin vs. Homebrew Champion, Ranger vs. Homebrew Hunter), in this case losing the required fluff link to the ocean while still capable of making a 'sailor' if you want one (in the same that you can make a 'druid' using shaman, a paladin using champion, a barbarian using fanatic, etc.). It fills the niche of the skillful adventurer with good combat skills midway between a fighter and a rogue that I don't think stock D&D really fills. The archetype might be 'Indiana Jones' which is hard to make with a core class in D&D.
I've thought a long time about the things I would import from 1e that are missing in 3e - simultaneous resolution, weapon vs. AC modifiers, casting times, facing - and for the most part I've decided that while those things work in 1e and are good things in and of themselves, the extra complexity that they would add to combat isn't worth the benefit. Third edition adds a lot of extra heft to the rules as it is and if we also add additional complexity we might be sacrificing too much in other areas for the sake of realism. The only 1e style rule that remains on my consideration list is casting time. I haven't got enough of a sense of the balance between classes yet to make the call, but if I get the feeling that the spellcasters are still overshadowing the nonspellcasters then I'll probably bring casting time back.
There are things right now from Pathfinder that are under consideration for importing into the homebrew rules, particularly the much clarified language surrounding combat manuevers and oppurtunity attacks and the changes to the turn undead rules. I won't import these whole sale necessarily, but the ideas will influence what I produce. Again, you can't necessarily cut and paste rules between game systems, but you can mine game systems for ideas about how they handle things.
For now, most of the influences of 1e on my game is in the style. I use to run open dungeon crawl at the LFGS using basically the random dungeon generation tables in the DMG. I still admire 1e's ability to use random encounters, number appearing, percent in lair, and random treasure tables to create an interesting sandbox. I still admire the feel of 1e compared to modern games.
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