I'm afraid your proposal will come with too many limitations. The current override conditions are pretty powerful, this is something you will not be able to reflect entirely in a template naming scheme.
I agree though that it would be a good thing to not have 2 directory structures anymore to search for templates (no longer one for standard templates + one for overrides), as this is confusing anyway. I guess this might break a lot of websites though, as they might have equally named overriding templates that are used only in certain conditions. Bad practice... but understandable because the template override system has a long (partially undocumented) history.
I do agree that the new scheme looses a lot of power, but if 90% of the existing cases can be covered easily, and 10% with ugly if/include-based-templates, I think it would be worth anyway.
Besides reducing the number of directories by removing the unnecessary 'override', the filenames for override templates would suddenly become standard. No more looking around for pagelayout_popup vs. popup_pagelayout (unless of course users go rampant with include).
Principal Consultant International Business
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