vaderios said:

Something like this for example..
Also see some schematics about the two cloud cities

Curse your sorceror's ways Darth vaderios, for making me want to throw in a few thoughts on all this, considering I've had some maddening computer glitches for a couple of days now, lol.
Anyway, I personally far prefer the 'thicker' proportions of the existing 'Cloud City', I'd have to say.
I think it's because it allows for many levels (and deep shaft areas) to be below the main city buildings on the top of it (which I prefer the look of too), as well as the extra layers of lighting around the main structure, as seen here -

Turns out it's actually a very different shape to the recent diagrams you posted here vaderios, and is a very elegant shape, just as it is, I reckon.
The actual 'stem' seems to be nicely 'tapered' too (with a 'ball'-shaped end to it, to my eyes), unlike the versions you've shown of it -
I'd have to say that this example of the movie version's basic shape is a reasonable representation, as far as diagrams go - http://img191.imageshack.us/img191/8033/2220bespin20cloud20cit2.jpg
....by the way, if anyone knows....can you please mention what that object actually is that falls down beside the 'stem'? Was it meant to be something that fell out at the same time as Luke? I certainly can't figure out what it could be if that's the case, as there's nothing obvious in the footage....
But if it's just intended to be a 'random something' from the City above to give a sense of 'vertigo' to the shot, I don't think it's required, as the movement of the billowing clouds far below do that adequately anyway. And as it seems to still be visible to us as it passes the 'ball' of the huge structure's 'stem', it must be a pretty BIG something! I reckon it lessens the vast scale that the 'stem' matte could indicate though....
Anyway, 'colour-correction' aside, I haven't a clue if Adywan intends to tweak any of the shots featuring the underside of the City's structure, or not....but I've never found any of the existing angles to be a problem, if things do remain as they currently are.
The reason why, is that I've always seen this sequence of events as happening somewhere near the central portion of the City's underside, somewhere not too far away from the actual 'stem' in the scheme of things....and although the structure does indeed 'curve' upwards all-around from this central point....I reckon it's such a huge 'surface area' to begin with, that any 'curvature' begins very slightly at this near-central point before it begins to get really noticeable further outwards....
....and I've always been able to imagine that we don't see the wide 'stem' in any of these shots, as the action is always framed in such a way, over such a small portion of the massive City's underside, that it's merely offscreen a little ways, due to the direction/viewpoint that we see during the relevent shots. For instance, I'd say it's very plausable for the 'stem' to be just offscreen from where Luke is positioned in all these following brief shots (where the 'camera' is static, although there is a slight motion of the clouds in the distance) -
1. Luke looks downwards, and across from himself a little, presumably at the massive 'stem' that is shown immediately after this shot....

2. This 1st 'long-shot' of Luke comes just after the 'stem' shot, so as he's shown to still be in the same position on his thin support, I always presume the 'stem' is offscreen somewhere on the right of the following shots....

3. And here's the 2nd 'long-shot' of Luke....

4. And here's the 3rd 'long-shot' of Luke....

5. And here's the 4th 'long-shot' of Luke....

6. And here's the 5th 'long-shot' of Luke....

7. And here's the 6th 'long-shot' of Luke....

8. And here's the 7th 'long-shot' of Luke....

None of the slightly different 'camera-angles' in these shots spoils the notion that the 'stem' is offscreen on the right-hand side of this huge surface underneath.
And although it's subtle, there's certainly some distant 'curvature' of the underside already apparent in each of these shots too. It's very obvious in the 7th shot down, but if you were to try lining up a straight edge against the structure's 'distant edge' in this bigger example of the 8th shot, then it's possible to see that this is the case with all of the above shots - http://img139.imageshack.us/img139/2584/screenshot124o.png
So I can certainly imagine that this action is located somewhere around the 'flatter', central location near the 'stem', which probably only starts to 'slope off' in a gradual way in all directions over the large surface area around it. Certainly, the fact that the distant outer edge seems almost level with the cloud level beyond, gives the impression that this can seem to be the case. So I reckon the existing 'angles' work fine without the need for any 'acute' ones needing to be added. (Also note that the breezy clouds in these shots above seem to be identical over the duration that we see them! Even down to the 'wisps' being the same, lol....)
Of course, there's a bit more variety in some of the other 'camera' angles during these scenes, that give a further sense of 'curvature' to the expansive underside anyway -
9. Such as this shot, where the Falcon is approaching Luke from the direction behind where we are looking towards him in the shots above. (Meaning that the 'stem' can still easily be imagined as now being offscreen on the left-hand side of the tiny Luke figure that Lando points to) Here's the start of the shot....

10. And here's the end of it....

11. And the same thoughts about an offscreen 'stem' can apply to this similar shot too. Here's the start of it....

12. And here's the end of it....

13. And the same can easily apply during this shot also. Here's the start of it....

14. And here's the end of it...

15. And as far as this shot of the approaching TIEs....well, they could be coming from anywhere that's in a generally opposite direction to where the Falcon approached Luke. I assume this, as the Falcon seems to eventually escape back in the same general direction that it initially came....certainly if you go by the lighting and cloud pattern seen in the top and bottom shots of the distant 'Cloud City' that I've posted here. Here's the start of it....

16. And here's the middle of it....

17. And here's the end of it....

Although the shot above is not a static shot of the underside in this instance, the long 'stem' can still be easily imagined to be offscreen somewhere due to the huge area that makes up the size of the City's underside. Like the previous shots, this one only covers a small portion of it surface, and so can tie in with that notion without any problems....
18. And the same goes for this next shot too. The Falcon only begins to move off from it's 'hovering' position at this point, after saving Luke, and seeing the approaching TIEs. It begins to veer to the right, and the long 'stem' that is presumed to be offscreen on the left of the Falcon would remain unseen. Here's the start of the shot....

19. And here's the end of it....

20. So the Falcon continues to turn around to the right, and we still only see a fraction of the overall surface of underneath the City....so the 'stem' would still be somewhere offscreen on the left of the frame. Here's the start of the shot....

21. And here's the end of it....

And although there is some 'camera-movement' during this shot above, it only moves to the right as well as pulling back slightly....so the long 'stem' on the left of the frame would still remain offscreen.
22. Our final look at 'Cloud City' is where the Falcon is now roughly heading back the way it came, to escape the on-coming TIEs seen in shots 15, 16, 17, 18, and 19. Now we can see the 'stem' again.... :)

So if Adywan doesn't do anything to the existing City underside angles, then they already work for me anyway, I'm glad to say. And just a couple of additional thoughts on some of these particular shots to finish off with -
1. Would making the likes of the red lights on the 'stem' and the outer edge 'flash' on and off, give more interest to the mattes, and 'sell' them a little more?
2. Is it possible to tweak the 'purple-ish' colouration of the City's underside that is seen in shots 9, 10, 11, 12, 13, 14, 18, and 19 above, so that they somewhat match nearer to the 'orangy-ish' colours of the other shots? (Not having a copy of the colour-corrected 'Empire', I don't know how they turned out in that)
3. This one's very trivial, but the little bit stuck on top of the Falcon's top guns doesn't seem the correct shape to me. Any chance of a minor tweak or removal there? Here's a bigger shot of it - http://img233.imageshack.us/img233/8671/screenshot122.png
4. You'd need to watch the scene in motion, but I reckon it would be neat to see a hint of the TIEs somewhere behind the Falcon in shots 20 and 21 too, as it seems that they have caught up too suddenly in the shots that come afterwards.
Meant to say that while looking at the various underside shots, all the antennas and protrusions seem to stick out at an angle relevant to where they are situated on the 'curvature', rather than all being vertical in relation to each other. So it seems they follow the angle of the curve.
Oh, and I've found a second 'flipped' Bespin shot now vaderios. I know how you like those. Watch this space. :)