Speaking of this, what would you say about complementing my 70D/35mm lens with an UWA or replacing the 70D with a 6D to get a wider (likely wide enough in most cases) view with the same 35mm lens?
Also, is 24mm usually enough in most cases for indoor/real-estate/architecture photography?
And, while we're at it, has anyone missed the "crop" factor when moving on to full-frame?
Whether 24mm (on FF, I assume you mean) is wide enough depends on the size of what you're trying to photograph, how much of it you're trying to get into the frame and how far away you can or want get from whatever it is - i.e., how "tight" the conditions are. Sometimes 24mm will be too wide, sometimes not wide enough (ditto when you're trying to benefit from the exaggerated perspective of a wide lens in other contexts). It's hard to be more precise without knowing exactly what you have in mind. Ideally, you could find out first hand by renting something. (E.g. Sigma 8-16 or 18-35 for your 70D or 6D + Canon 16-35 or 17-40 or 24-105.)
If you get a 6D to supplement your 70D, you will be amazed at how much less noise you see in low light/high ISO images (among other things) and in many situations may well find yourself preferring the images you get from a 6D, leaving aside crop factor considerations (most if not all EF lenses work better on FF than APS-C bodies).
Hard to say whether you would miss the crop factor either; depending on the lens, cropping a 6D image may result in an image at least as good if not better than the uncropped 70D equivalent, though of course that doesn't help with framing. I've been wondering whether I need to think about a longer native lens for the Sony A7r I recently bought, but the combination of its high resolution with the remarkable sharpness of the Sony-Zeiss 55mm 1.8 allows so much cropping, with excellent results, that I don't feel any particular rush to get anything longer. (I have an APS-C Canon, but I don't think I've used it since I bought it because, good though it is, my FF Canons make better images. On the other hand, I use my Olympus M4/3 quite often, and it of course crops more. Who knows how you would react....)