From what I can gather, one challenge with 24V and higher is that switched-mode converters, such as the buck converters used in the power stage, get a lot more inefficient when operating at high ratios.
You can see this effect in figure 6 in this[1] application note, where it's >90% efficient at ratios down to 10:2.5, but then drops to ~78% at a ratio of 10:1.
So if one goes for higher voltage perhaps 48V would be ideal, and then just accept the GPU needs a two-stage power conversion, one from 48V to 12V and the other as today.
The upside is that this would more easily allow for different ratios than today, for example 48V to 8V, then 8V to 1.2V, so that each stage has roughly the same ratio.
You can see this effect in figure 6 in this[1] application note, where it's >90% efficient at ratios down to 10:2.5, but then drops to ~78% at a ratio of 10:1.
So if one goes for higher voltage perhaps 48V would be ideal, and then just accept the GPU needs a two-stage power conversion, one from 48V to 12V and the other as today.
The upside is that this would more easily allow for different ratios than today, for example 48V to 8V, then 8V to 1.2V, so that each stage has roughly the same ratio.
[1]: https://fscdn.rohm.com/en/products/databook/applinote/ic/pow... (page 14)