That's an interesting thought, perhaps bit "tricky" in some ways, but definitely not wrong.
I am a big fan of consistency, because I think consistency makes any app easier to understand and to use. So, if this idea is adopted I definitely think it should be applied consistently to all the main GTD lists - i.e. not only to Someday but also to Next, Waiting and "Scheduled" ("tickled" items). Nothing wrong at all with the idea as such. In many ways what you suggest would be useful.
But there are some considerations that would need to be made quite carefully. It needs to be possible to see clearly in the Next/Waiting/Someday/Scheduled lists whether a certain item on the list is in fact a project (a "node" with sub-items) or a regular action. And for the case that both the project and some of its actions would belong on the same list there needs to be a decision (a default, a user preference setting and/or a toggle) controlling whether both kinds will appear on the list or just the actions or just the project, and maybe (?) this even needs to be controllable on a per-project basis (??, I have not thought much about it). And a decision (user setting?) for whether the projects should appear in mixed order with the regular actions or as a separate group (of projects only) either at the top or at the bottom of the list. And maybe some users want this feature turned off completely (I don't know). Etc.
But it is definitely a feature worth considering, IMO.