SA1124 DoNotUseRegions suggest that region should not be used anywhere. Is it really reasonable?
I think region is a way to group relative code together and make large class easy to read, for example, if you generate interface method in for class, a region will be automatically inserted by visual studio.
I would like to remove this rule while checking code style. May I know your ideal on this rule?
This is going to be a personal preference thing. The only thing that matters here is what you and your team prefer. Forget what style cop says, you're the ones reading it, you're the one's maintaining it, whether with or without regions works better for you, that's all that matters.
If you're releasing it as an open-source project...and this is my opinion here, I think the same applies. Use whatever the core team is more comfortable with. If you get a lot more team members aboard and more contribute, re-visit the issue later, this can always be changed.
I think that regions can be abused, but they are a useful technique for allowing a reader to focus on certain areas of the code at a time.
I would avoid too many levels of nesting, however.