I hope iOS 9 introduces a Dashboard. These widgets make absolutely no sense in the Notifications pull down. Dashboard would be great to the left of the first Home screen (where Search used to be).
The idea behind having widgets in the Notification Center is being able to quickly access them.
Though right now the NC is sort of bloated with all the widgets and the display of relevant information for the day (weather, reminders, calendar events, etc.), I don't think putting those widgets to the left of the first home screen would be the best idea, in practical terms.
Let's suppose that you just opened a certain app inside a folder on the second page of your home screen, and from there you want to open an app in the NC. Right now you just swipe down from the clock or top bar, tap the widget and boom, you can open mostly any app from anywhere in the system with just two touches.
Now let's suppose that we face exactly the same example, but with your suggested method. First you would have to press the home button to suspend the current app, then press it again to close the containing folder, then press it once again to go to the first page of your home screen, and then swipe to the right to access that new widgets page. Such method obviously would imply widgets to lose their purpose of being quickly accessible.
The only viable thing that I can think of, to implement something like you suggested (which, by the way, I think is absolutely logical from a UI perspectiveor at least makes way more sense than putting actionable shortcuts or apps in the Today view of the NC, functionality aside), would be to add some new system-wide shortcut using the home button or a Touch ID button, perhaps something like pressing it three times. Damn, they could it even implement Force Touch on it, so now we have way more possibilities, including the one that I've just imagined. That way widgets would be even more accessible than they are now.
Unless some of the ideas that I've described in the last paragraph were implemented, which I doubt, I think the best and simplest way of solving the problem in iOS 9 would be adding a third tab in the NC called Widgets.