Well, I suppose I shall throw the second stone into the pond after Ringwaul hit the target already.
I hope I'm able to express all my ideas properly, it's English at last :). Should I fail in it, I'd post in the German forum, hoping for someone to translate it afterwards.
Actually the idea came some days ago (as always, some minutes before falling asleep...) but I haven't found the time to post it until now.

So, what to think about the Subject above?

Brief Description:

Modularity: My idea is, that the production/construction lines in OC will be modular, meaning each building/item could look/work different, depending on how you build it.
Entering Buildings: I hate the idea of a clonk standing before a castle and grabbing it (Clonk humps castle?!?). I would propose a system basically first mentioned here, but with some differences.
How to build: I think in OC the "press down and wait"-sytem is outdated, we should come with something fresh. How about placing the carriers (German: Stützbalken, Grundgerüst) in a pattern and styling it afterwards?

A little more detailed:

Modularity:
I think someone already proposed it for items, but I would go a step further and say: Everything you can somehow "build" in OC should be modular and combining it with other items/buildings would create something new. Of course, the top-most items in this "Combining-Tree" can't create something greater BUT because of the community-driven development that is a big part of clonk, it should be made possible to combine even these.
And yes, I know this is difficult to realize, but it would be a fresh new way of seeing Clonk and it would clearly define the autonomy of OC.

Entering Buildings:
There are huge discussions on the forums about how to manage this topic and we all know the balancing issues that came with the way of handling this in the previous games of the series.
I already said I'm not a big fan of PeterW's idea, because I can already see Clonks standing in front of a big building and "grabbing" it. That would be ridiculous. Only to cite PeterW himself: It would be like "reinventing Clonk - and killing it in the process."
So I thought about it sometime came to a solution and saw, that it was posted by pluto already, but with little differences and limited to castles.
My idea: Every building should be accessable, not like in CR, but with some kind of layering. It basically works with 2/3 layers, you enter a building and the inside of it gets visible (for fluidness I would include a small fade inside-outside) and the fog of war surrounds the walls of the room, except another of your Clonks stands e.g. in front of the building or a door to the next room is open. We should even go as far as including windows to the walls and compute the FOW throughout the layers, so you can spot enemys from the inside, but also from the outside (the indoor-Clonks should then have an alpha-value of 50% so that they can be distinguished from Clonks standing on the outside), to make "shoot-arrow-through-window" possible.
If we could realize it like this, the hiding indoors problem will be solved, because meele WILL be possible in buildings.
The 2nd layer also has to allow "buildings"/minor structures to be placed (e.g. the anvil, wall tapestry, in general customization) which leads me to my second idea

Building:
I have rather complex ideas for this, all of them in a state you could call "alpha". (nerdexpressions ftw!)
The keyword is, as it is with all of my ideas in this thread, modular.
I will explain my ideas with the example of something simple, let's take a wooden hut.

Step 1: You need construction blueprints of the modules you want to use. So this would be "blockhouse" in this case. As it is a rather simple one it is available from the start, although (!) the map creator can turn it off, if he likes to. Well, until now it's like in CR. But here comes the difference: When you pick your hammer and stand at the position you want your hut to be, you press "e" and the "hammer menu" appears. It basically consists of 2 sections, in the first there are all the building materials you carry, in the other all blueprints you researched, sorted by kind. If you don't know the blueprints by heart, you click on it and it unfolds in a seperate window. Now you click on the building material you need, in this case a log, and your cursor gets that log, that you can place within a certain radius around your Clonk, at any pixel you want, with the restriction that it has to touch the ground (right, it's a building after all...). You can rotate the log with w/s (vertical), a/d (horizontal) and q/e (diagonal). After you placed the first log you pick your second one, but this time it moves snapping to a grid, that is relative to the position of your first placed material. The blueprint is also gridded and shows how to arrange the materials. After you are done you click the button "done" in your building menu. If the construction is valid (within the researched blueprint) the sign in front of the construction changes and you come to step 2. If it is invalid, the structure crashes and all materials fall to the floor (maybe a loss of 1/10 and at least 1?).
This would make no sense, if that was all, because it would be basically the old system with a repetetive manual construction. BUT this isn't the cool thing about it. As the gridded space around your material is only limited by the nature around it, you can continue contructing after you have added all the needed materials for the "blockhouse". Want a second room? Just continue and place a second blockhouse, not doubling the connecting wall. But you would need a door for this etc. (I know it is hard to program in the first place, but blueprints and possibilities can be added step by step by the community developers).
Also there need to be multiple possibillities for one blueprint. You can do a wooden floor or a floor consisting of clay (German: Lehm), there are wooden roofs and roofs consisting of hey etc. (rhyming power!)

Step 2: Now that you have your rough building standing, there's an optional step available, customization of your building. I don't relly know how this should look, but individual buildings would be really cool. Maybe there could be a spray gun to color your walls or different floors etc.

Step 3: You all know this step. The computer calculates how the building will look (would be hard to program with all the possibilities, I know) and you swing your hammer. The question is: Auto-build (arrow down) or holding the left mouse button?

Step 4: It has to be possible to expand a building. I have no idea how this could work, maybe holding the hammer, pressing "e" in front of a building, menu appears ("expand/change","destroy"), you come back to step 1 and are able to add/change something, click "done", the valid parts of the building are left standing, the others fall apart, and in step 3 only the necessary parts are rebuild. Maybe.

Phew, that was a mass of ideas and it took me >2h to write it all down, so I hope at least something of it goes into the game :)

Edit: Because a lot of people asked why the system would be more fun, I'll try to point this out:

The construction system:
1) I hated how every hut, house etc. looked the same in CR/CE/CP. When you needed to get a lot of points for settling, you had 10+ stone houses or other buildings in a row just to reach the goal. With the modularity system you would be able to create a building that really looks like one and gives as much points as your row of huts/castles(the building-like ones). Like the old castle sytem (the modular one) with outer walls and a real difference between outside and inside.
2) If you define a building not as "type of building" but as hull, you can assign different behaviours. For example: You build a stone building with a funnel in the roof. If you place a forge under it and throw iron and coal in the funnel, clean iron will be produced, but if you place a generator under it and add a wire, it'll produce energy. And if you add another room with a funnel to the building, you could produce iron on one side and electricity on the other, in the same building. If you add a tube to the ironforge, you could automatically transport the iron to your base after the process etc.
You could compare it to redstone in MC: A simple mechanism, but you can construct complex ones with it.
3) The system isn't necessarily slower than the old one and in Meele you don't have to build complex buildings if you don't want to.

The building system:
This really is obvious: Indoor fighting! Running though an individually styled building (not necessarily a castle!) and fighting each other with agile Clonks, I can't imagine something more fun! There could be doubled-sized hall modules, staircases etc. that would make it worth constructing for meele. I also like the idea of lockable+destroyable doors.