Okay, I still use DSKRs, 7D mkII, 5Ds, and some other crop bodies, but here is what I think Canon needs from a nature and macro photographer's perspective. As well as the usual long fast telephotos, I think Canon need something in the class of Sony's 200-600mm f6.3 and the Nikon 500mm f5.6 PF. These are both lenses which have changed how nature photographers can shoot. There are no Canon equivalents, and really Canon should learn from how well these lenses have been received. Canon also needs a long macro lens in the 180-200mm range, because the EF 180mm f3.5 was already a very outdated lens, which really needed to be updated with IS and faster AF. Probably an RF replacement for the unique MP-E 65mm would be useful. I'd recommend starting at less than 1:1, say 1/2 or 1/3 life size, to make it more versatile.
This is in addition to the lenses we are likely to get i.e. normal macro lenses and long fast telephoto zooms. I think one thing Canon needs to consider, which would be especially useful for macro photographers is greater than life-size, say 2:1 like the Laowa lenses. This could negate the need for an MP-E type lens. This massively increases the versatility of such lenses. Having to remove macro lenses in the field to add extension tubes is far from ideal because this massively increases the risk of getting debris like pollen grains on the sensor.