It's not only the off-axis response of each separate driver you should look for.
The goal is a consistent off-axis/power response of the whole box, getting as close as possible to constant directivity (having a flat response at each angle off-axis). This means either omnidirectionality (same response in all directions) or a uniform fall-off as you go off-axis. Thus, drivers with a large dispersion are not necessarily always the best - it all depends on what you want to achieve and how you combine them.
It has been pointed out that true CD cannot be obtained with piston sources(cone/dome drivers). It is nevertheless important to try to get as close to that as possible.
I wonder how a dome midrange paired with a ribbon would sound. I guess we would have seen that quite often in the eighties if they had good and affordable ribbons at that time. But since dome midranges have gone with the wind you were probably one of the first to try.
Driver with good off axis response will normally have bigger sweet spot. If you don't normally listen to music on one spot, good off axis will be something you looking for. However, top quality driver from Seas and SS don't have good off axis response. As always, there trade off.
Good 60D off axis with flat response driver is hard to find so far I only found these.
For midrange Seas 4in MCA11FC, if you can find it. For midbass, Peerless 4in 830854, 830870.
For tweeter, have a look at OWI and OWIV.
The best is download the catalog for each manufacturer from Madisound. If is easier to find what you looking for.
..Tilted response on-axis though. And I believe it was tough to find the 8 ohm version when I was searching for them, if I remember correctly. I was only able to find it in 16 ohms. Would probably make for a good MTM.
I agree with the statement regarding the Hiquphon 3/4" tweeters. Supposedly some of the best off-axis response to be had in a soft dome.