The 7D is a specialist for wildlife and sports. The most obvious pick for a successor would be the 7D2.
I would definitely and strongly advise against the 5Ds unless you want to completely change your photograhic focus. The high resolution of the 5Ds very quickly diminishes at higher ISOs. It thinks its the classic case of an overengineered product.
I would also advise against the 6D. Its not a bad camera at all, but it has a rather simple Autofocus. Not what anyone would want for wildlife and sports.
I see no real issue with the 5D Mark IV or the predecessor Mark III.
If you want to stay with Canon, my personal favorite would probably be the 1D X. I dont know how much used prices have fallen for this one now, but that would be a great performer.
If you move to Nikon for "you can just magnify", you probably talk about the Nikon D810. The D810 is a great all around generalist camera and can be used for sports as well, including having an top of the line autofocus (when it was released, anway, not the new gen that came out with the D5 and D500), but overall its more optimzed for towards high resolution and high image quality, not fps and buffer and high ISO. And yes it has 36 Megapixels at full frame 36x24mm and quite useable 15 Megapixels at APS-C 24x16mm, so that part isnt wrong.
The D750 would be another option for this. It has basically the same autofocus (slightly better actally, which can be very visible in certain extreme situations; for example it can handle lenses with a maximum aperture of f/9, while the D810 cannot handle more than f/8), not much higher fps and not much more buffer either. Its cheap though and its a very fun camera to use with its flipscreen, small size, good grip etc.
My personal favorite for your selection of photographic themes would be the recent masterpiece from Nikon, the D500, with the AF-S 200-500mm f5.6 VR and probably also the brand new Tamron 70-200mm f2.8 VC "G2". That would be an extremely fun combo, at 10fps for 20 sec (with fastest XQD cards; the 20 sec stop is software limit only, you just have to release the shutter release for a moment, then press it again for another 20 sec at 10 fps), AF over all the image area (only small bands on top and bottom of the landscape view arent covered), extreme autofocus performance and extreme high ISO (though the really useable ISOs end around 50k). That would already be about $4600 though.
Obviously there would also be the D4s and its predecessors.
I would advise against the D600 and D610 for the same reasons as the Canon 6D: rather simple autofocus. I mean OK its better than the 6D, and I had that camera and its not awful for sports, but there is better out there to have.
Oh, and switching to full frame you probably want one of the extreme telephoto lenses out there, like the Tamron 150-600mm f5-6.3. Or the AF-S 200-500mm f5.6 VR. I understand that Canon has an extremely expensive 200-400 with builtin 1.4x extender, but my guess thats not an option (and for that price I'm not conviced its worth it, either).