At the moment, you can offset a plug-in from it's default order by incrementing it.
order = pyblish.api.Validator.order + 0.1
Orderings are assumed to fall within certain ranges.
0-1: Selection (default 0)
1-2: Validation (default 1)
2-3: Extraction (default 2)
3-4: Conform (default 3)
Which means that there is no apparent way of decrementing.
"""I am now indistinguishable from a Selector..."""
order = pyblish.api.Validator.order - 0.1
So I was thinking whether it would be a good idea to slightly offset each default plug-in to lie at the center of it's range instead of at the beginning.
0-1: Selection (default 0.5)
1-2: Validation (default 1.5)
2-3: Extraction (default 2.5)
3-4: Conform (default 3.5)
Which would make it possible to arrange a custom plug-in both before and after any defaults. Considering the otherwise large change just done with 1.1, I think the change should either happen now, or at 1.2, possibly 2.0 depending on how much it affects anyone.
I've done a quick test myself, and I haven't offset any of my plug-ins further than 0.2, 0.3, which means I'm in the clear. But if you have offset it > 0.5 you will have to update your plug-in.