House rules....
For us, the house rules that are already being designed, are stuff that are pretty much rules independent.
The reason I say this...we did tons of house rules/custom classes/races in 1E, 2E, as well as 3.XE.
Many of them in 3.XE were done without knowing what the official rules for that aspect were. DId it hurt anything? nope. Was it hard for me to balance or DM? nope, not at all.
Our experience shows that races aren't that a big deal..really, if one race has a +2 dex, the other has a +3 int...it really doesn't change much. Then also, based on the type of campaigns DMs are experienced in DM'ing (me is mainly planescape, incorporating all settings, OR strict dark sun). FOr example, my current campaign is the 3rd time I'm dm'ing a character that can fly naturally (A Lillendi)...haven't had any issues with it so far... and many fun situations have occurred due to it so it's all good.
Classes - I am currently writing up new versions of the WIld Mage (a friend uses one), a Chronomancer (time is very important in my games), Far Realm (also used more in the past year) stuff, wizard and cleric. These are rules independent since it goes back to the very basics of what WotC said themselves of classes. How do you envision monsters, and make them like that, without all the excess rules, components, skills, etc. We did that for the classes, and people seem to agree, that for us, the wizard, cleric, and espec wild mage need changes. The wild mage is the furthest along so far, and then I realized, my concept is very similar to the one in some other d20 supplement. Not exact, the basic concept is the same. And that fits into how I do my planescape cosmology perfectly.
Other rule changes that are totally transparents to the game system are stuff like the critical hit system. Assuming the game system has a hit/miss mechanic and thus critical hits are technically/logically feasible, you can make minor changes as you see fit. Really, as long as everyone understands the extra threats or maybe even a more leniant system, it can be handled without breaking a game.
I think it all goes down to how many house rules/custom rules is the DM willing to help create, and the players willing to handle.
Right now in our 3.X game we have:
1) 2E C&T/S&M crit hit system
2) localized healing, called shots, and limb damage
3) customized item damage/destruction system (almost liek a combination of 2E and 3E to make our own version)
4) rules for super detailed combat (that takes into account hitting but not penetrating armour, clean hits, missing totally, and armour damage that can accumulate from taking hits without them actually penetrating, etc) -- we don't use these though since they take alot more time compared to the value. the only place we decided where it be fun is during a duel
5) customized advancement for higher ECL races that also have a class associated from level 1 (basically we advance like 2E method, so the 'race/species' and 'class' advance together, since in real life, a person grows as a person, but at the same time they get more educated or get better at a job. yOu don't grow to 50 years old first, and then start advancing career. Friend's sister liked this better for her Lillend.
6) monster changes - mainly for planar creatures only..actually come to think of it..ONLY for planar creatures.. DR is modified a bit, sometimes easier, sometimes more like 3E which is harsher. some pure immunities back based on power of creature. spell lists modified. some spell powers/frequency changed back to 2E version where we think it fills concept of creature better.
7) cosmology changes from great wheel to our own planescape variant..still has all planes and we've already incorporated Feywild and Shadowfell with custom rules for what we know of them now.
8) changed a bit of how magic works in sense of how I believe/use the cosmology of D&D as a whole to make what we refer to as, "The Universe of D&D" . Nothing major yet since there are some classes that fit this vision better so I allow players (for a higher ecl cost) to use those variant mage types instead if they wish.
9) races & classes...if it doesn't exist or a friend wants something that is different enough that it can't be done with anything that exists currently, will make it. Not a problem. Spent a few days with a friend making a custom planar class..but he ended up just making a wizard when he realized he hadnt played a wizard in a while...go figure.hehe The class seemed interesting, so may introduce it as an optional 4E one with some more changes to bring it up to the 4E per round/day, etc action system..but for that will wait to see how it's done.
10) many spell changes so won't mention and many item changes as well.
11) toughness using 4E version in current game and thinking of using the 4E -ve HP system
For 4E:
1) wild mage - totally differnet system... similar to a combination of 2E and the non wizards d20 wildmage + some homebrew ideas and a bit of the 4E sorceror concept thrown in. Player already loves the idea and party says it sounds very cool and they are looking forward to it. Drawbacks..won't use the perday/per encounter, etc system as his powers are very different and in many ways, totally random or close to it...but he likes that and says all 'wild mages' they make aren't as chaotic/wild as he envisions. The magic system is what I use for many demons now, so it works quite well and makes for VERY interesting encounters...
2) wizard - harder to do..but the concepts are typed up already for what we want. may end up delaying it until we do our mass character conversions to 4E and then introduce it to the wizard player. That way, we can always review the new wizard firstif we wish. AGain, our concept has some at will powers and rituals, we've always used for stuff like very powerful spells (Gate, wish although players not high enough for this stuff)...but in terms of house rules. Balancing what we envision has been decided. I think it will work out fine due to my players beliefs on how they want a wizard to be. I am looking forward to doing this entire write up. They will not have per encounter stuff...again, players are happy with some classes working differently. For them overall, they just care that it's fun and it brings about better roleplaying possibilities or the concept fits with what they envision better.
3) cleric - never been happy with cleric. I just don't envision how they are now HOWEVER, 4E seems to be fixing it. I will probably write some stuff up , but only use it if 4E doesn't take the class far enough. Druid will have a few changes, again based on what we envision.
4) cosmology - have taken the expected 4E list, and added 4 more gods that we wanted. This helps fit into our "universe of D&D" and a big change....why should only clerics get powers from gods? in legends...basically all religions (christianity, hinduism, ancient greek, etc) have stories of non-priest types that are 'blessed'. I am writing up stuff where any class can get varying powers if they want a god; which varies based on their class and the specific god. The issue is, what to give players who don't want a religion. THis I started but still thinking on this. Players are all VERY excited about this... the key is, this is meant for hardcore roleplaying groups. They really have to roleplay their characters beliefs when appropriate for their gods to show these benefits to them...so they could get more or less even.
5) we will use current critical hit system that we use now, along with various spell changes, and item damage rules as we do now. These can be added to any system really. We already use warhammer style templates and even the Warhammer scatter dice for random grenades...works out great.
6) as my party has a Bard ,half orc, Lillend, Barbarian, swordsage, wildmage and a lycanthrope infected player, many house rules will be used in our conversions..again, the basics of what I have seen in 4E from reading everything on the site so far...not worried...half orc will be easy enough...since stat bonsues really aren't that big a deal. The players said they would just like to use the existing 3.5 stuff as it is now until the PHB 2 or whatever material is released. I am ok with that, and we agreed on changes on the fly if needed. All in all, after 13 or so years of Dm'ing...not an issue.
7) using our current "universe" of D&D but all 4E planes will be incorporated. Far Realm will be kept more hostile as it is currently..and insanity has also played a role in our campaigns..with a couple players having to roleplay it, and me messing with them to help them along. It made for great acting experiences..and very funny moments.
8) monster changes, many as we do now, and some going even more extreme than what they did to get the monsters to fit their concept better. THis will use a lot more custom ability/spell tables that are in high level concept right now.
That's all I can remember now..I'd have to look through my campaign folders on the laptop to find everything...
I think most players here will think we are crazy with all the cahnges we use currently and am planning for the future...
Sanjay