You have a 24-105mm, so I imagine you'll want wider than 24mm...
"...the 15 is too wide on the 5D2..."
First of all, there is no point comparing your 8-15mm Fisheye with a 16-35mm or any other ultrawide zoom. They are completely different animals. A fisheye lens makes no effort to correct for line curvature and at 15mm on a FF camera, the 8-15mm renders a 180 degree angle of view. In comparison, the EF 16-35/2.8L II or Tokina 16-28/2.8 AT-X are designed to correct much (but not all) curvature and at the 16mm setting renders a 108 degree angle of view. 108 degrees vs 180 degrees. The Zeiss 15/2.8 ultrawide (non-FE) might be another example, offering 110 degree angle of view. So just by it's very nature, even at the same focal length a non-fisheye lens is far less wide than a FE lens.
Your "wants" for this lens are pretty much impossible to meet. You're going to have to compromise somewhere. But there are quite a few possibilities, if you are a little flexible.
The Canon 16-35/2.8 II uses an 82mm filter. It also is not critically sharp edge to edge (few zooms will be).
The Tokina 16-28/2.8 AT-X is not able to mount any filter, it has a protruding front element, much like your 8-15mm Fisheye.
The Zeiss 15/2.8 also has a protruding front element, plus is manual focus only. This is probably the sharpest ultrawide you can buy. But it's very, very expensive.
There also is a Zeiss ZE 21mm f2.8, another critically sharp lens. Manual focus only, but it can be fitted with filters, tho it requires 82mm.
Canon 17mm TS-E is also extremely sharp, but very pricey, cannot be fitted with a filter due to a protruding front element, and is manual focus only. Plus it's f4, not f2.8.
Canon 14/2.8L II is the widest available non-fisheye prime, giving 114 degree angle of view, pretty darned sharp but is rather pricey and cannot be fitted with a filter due to protruding front element.
Canon 17-40/4L IS is possibly the closest to what you want, can be fitted with a filter and uses a 77mm, but is f4 instead of f2.8, and isn't completely sharp from edge to edge of the frame (again, very few zooms will be).
Canon 20/2.8 USM is reasonably priced, can be fitted with a filter (72mm), but isn't razor sharp edge to edge, especially wide open.
Rokinon/Samyang 14/2.8 (also sells under Bower, ProOptic names, and as a 13mm Vivitar... same lens in all cases), is very affordable, but cannot be used with filters due to protruding front element. Manual focus and manual aperture only. Good copies are surprisingly sharp edge to edge (check for decentered element... which will show up as a softer image one one side than on the other), however this lens has strong "moustache" distortion in the middle. Might not matter for scenics, star trails, etc, but can be a problem for architecture, for example. Distortion can be corrected with software (PTLens and others).
Sigma 20/1.8 is a larger lens, using an 82mm filter, and isn't terribly sharp edge to edge.
Sigma 12-24 is the widest non-fisheye zoom available for full frame. It's going to have some distortion, isn't the sharpest of lenses and has an f4.5-5.6 variable aperture. Can't be fitted with filters, protruding front element and built-in hood. But, it's the widest available (122 degree angle of view), short of going to a fisheye lens.
Some like to adapt the Nikkor 14-24/2.8 for use on Canon.... It's a fine lens, very sharp, but adapted for use on Canon will be manual focus and manual aperture control only, and it cannot be fitted with a filter due to the protruding front element.
There are probably some I'm forgetting... those are just the lenses that come to mind.
Note: some of the lenses with protruding front elements might be able to be fitted with a clamp-on adapter allowing the use of square and rectangular filters, such as Lee. There are adapters made for this purpose, for a few of the more popular lenses. Or a homemade adapter might be possible. Some of those lenses provide a rear slot for gel filters.