I don't see why any additional level of granularity is needed? You'll know they're social challenges because the DM will call for social-checks, or checks with social outcomes. You'll know they're environmental challenges because the DM will call for checks that are relevant to the environment.
I'm not really aware of any improvements made by the community, again, I find 4E to be the least-needing of house ruling of any edition I've ever played (unless you're aiming for a specific flavor of gaming). That is to say: I think Skill Challenges work pretty well out of the box.
IMO, yes and no. We basically got two approaches to skill challenges from the publications. The first one was a list of skills and then correlating actions based on those skills, which you see in DMG1 and DMG2. Then later (example of this is in the Cairn of the Winter King adventure in the Essentials Monster Vault) you get a second style of skill challenge where they had a series of possible situations that the players come into and how the skill checks can be used for these situations. I personally found the second model to work better for player immersion into the role play. You also see the evolution of skill challenges throughout the books and there have been videos online from DMs where they talk about different approaches to skill challenges. PHD20 is a example. He has a whole series of 4E videos that players and DMs can reference.
I don't know about your experiences, but 4E is pretty modular. I'm not sure how to make it "more modular" without simply creating more abilities with more variety in their effects.
I am not talking about making it more modular, I am talking about bringing the modular element back to the center of the character development and streamlining the process in a way that is more clear to the players what is going on. They moves away from that with Essentials and started coming up with linear class structures. I would also like to see the builds focus more on the roles then just the classes. For example, with the fighter, the mark ability was a key feature of the fighter being a defender. Have each build have a different Mark ability that reflects what they build is doing. So the Guardian Fighter would have a mark ability that strengthens his ability to protect others, vs the Battlerager Fighter build, which would have a mark ability that strengthens his ability to cause massive damage to a single target (like a striker class).
Yeah, social "encounters" have always been something D&D never handled well, but I can't say I can think of a system with social encounter rules that isn't A: really complex or B: not good. Social encounters are really free-form and depend a lot on a player's ability to think critically and speak well, or at least roll well to make their character speak well.
True, but you can still come up with a social encounter system that can still drive the narrative forward in a meaningful way. I think that is something that a lot of players get confused on. You don't need a social encounter system to have every day roleplay interactions. You need it for situations where you need to convince a NPC to do something that they might not want to do or be involved with game stories with a lot of political intrigue. Yea, you could just have it made up on the fly, but you do run into players who want to use more strategy in their interactions and having a system in place would help with that. The folks at WotC saw this at one point and came up with a basic social encounter system in P2: Demon Queen's Enclave adventure.
the social encounter system I came up with ended up being really similar to the one in P2: Demon Queen's Enclave (developed independently) and my players have been really enjoying it, especially our bard and rouge. They were able to shut down a combat situation with a Frost Giant with them and they were able to get the giant to make something for them through it. Feel free to check out the rules in P2 if you want. The one I came up with goes along the line of there are certain "social actions" (like basic combat actions) that you can do in a negotiation and these are played out differently depending on whether you are trying to be honest (diplomacy), cheat and lie (Bluff), or if you are trying to bully your way through it (Intimidate). Depending on the NPC, some methods work better then others, and one player could do the same social action in different ways and get different results depending on how they go about it. I took these actions and then I used the skill challenge rules to come up with my social based skill challenges. They also ended up correlating with the Bard's AEU abilities really well.
All I'm really getting at is that, at the end of the day, there's IMO so little to justify a 4.5 that at best you'd be getting a 4.1, and I question if that's really necessary for a whole new retroclone to be born out of.
I mean, about 2 pages of houserules would probably address everything you just mentioned.
2-3 pages of house rules, a 5 page pack on social encounters, a 10 page booklet on the Fundamentals of 4E magic, and about 40 pages of homebrewed skill challenges and narrative challenges. I have been running 4E games since 2009 and there are other changes I would make with things in the math and such, but frankly I do not have the time to do it on my own. We are talking about what we would like to see in a 4.5, so I threw my 2 cents in.