I also think a relative value on snapshot changes would be a good default. Perhaps with a setting to toggle to absolute values but that’s perhaps going too far into unneeded complexity.
Well, if we talk about the HX series, there might be an issue with presets created on the OG units not being compatible anymore if the snapshot paradigm would be changed.
However, for some situations there'd be different ways to tackle the issue. Let's take reverbs and delays. Even if snapshots were still using absolute values, you could slap them onto a parallel (mixed in) path and control the return mixer value globally.
Anyhow, in the end, for my (very personal!) use cases, I'd actually wanted anything global to be "truly" global. Read: I likely wouldn't use snapshot control on these blocks (regardless whether it's relative offsets or absolute values) at all.
While there are a whole lot of implications thinkable, for me (again: very personally), the main idea behind global blocks would be for them to allow me to adjust very crucial parameters quickly and systemwide. And it's just those very crucial parameters I would never even think of controlling them via snapshots. Just as I'd never wanted an analog amp's treble pot to be altered on a preset. If I wanted that kinda thing, I'd possibly rather use an EQ in addition.
As said, this might only be me and there's several more scenarios for global blocks to be useful, but as is, I could perfectly live with the limitation of snapshots being snapshots and global blocks being global blocks. I would even be fine with global blocks overriding all snapshot values.
Yes, there'd be better solutions, but nobody would force anyone to use global blocks, so people could continue doing things as before and only use global blocks when applicable.
If you think about all that stuff in a kinda streamlined fashion, such as "which parameters do I actually want to control via snapshots and which parameters would I want to control globally?", you might perhaps come to a similar conclusion, namely that there might be very little overlap (possibly with the exception of delay/reverb parameters) between them. For me, that very certainly is the case.
Add to this that there might be other "handmade" solutions. Want more gain from your amp in one snapshot while the amp is set to be a global block? Add a boost in front and control it via snapshots. Want more delay on one snapshot and still control the overall mix globally? See above, control the delay via snapshots but set the return mixer to global.
As pretty much everyone, I think it's a wise idea to keep things more or less straightforward. Adding additional layers of complexity, while being fantastic in nerd-land (such as the mentioned offsets) may not click well with many people. Let alone they might become difficult to implement.
Adding a new "you can now adjust this block globally but lose snapshot control" option however, as limited as it might be, would be pretty straightforward.
Sorry for all the words (I know, some people don't like my diatribes...).