We already made good progress and I think it's time to ask some other modders for feature suggestions comments.
I'll also give some more details on how the framework is organized, and how it works:
The basic idea for adding custom widgets is to utilize the loadMovie ActionScript function. It allows loading of an external .swf from the Data/Interface folder into the current movie (menu) at runtime.
So we added a widget loader to the HUDMenu, which is sent SWF names from Papyrus, loads them and adds them to the HUD.
Examples for widgets are labels, a meters, and icons. But it can basically be anything, with the restriction that they can not receive input (because no HUD element can).
They are just there to display information.
The first benefit of this is that it's no longer necessary to modify HUDMenu directly if you want to add something. Instead you can add a standalone .SWF and include it with our loader,
so there will be no potential conflicts with other mods that do the same.
The next step is to create several general-purpose widgets that can be parametrized from Papyrus. This is one of the things we could use some input on.
As mentioned, currently planned is default stuff like labels, meters, icons etc.
Our current progress is that the loader and the Papyrus infrastructure for the framework are pretty much done.
In Papyrus, a widget will be represented by a custom quest you create in your mod. Based on the widget type, you assign the appropriate script to your quest, and then you can interact with it by
setting properties or calling functions on it.
What's working so far is a generic base widget, which has the functionality all other specific widgets will extend later - position, visiblity, alpha value, scale etc.
A screenshot of what that looks like in the editor:
http://i.imgur.com/kX5M7.jpg (type = test means, it will load test.swf from Data/interface, which is just a simple red square)
And in-game:
http://i.imgur.com/IvVze.jpg (as you can see, I set different horizontal positions and alpha values for each test widget).
So while these are just useless red squares for now, they have been added without writing any code. Just creating the 3 quests, adding the script and setting the properties made them appear.
Of course these properties can be updated as well. So if you set SKI_ExampleWidget.Y = 500 in another script, the widget would move accordingly. The widget state is saved and restored on reload.
Alright, so if you want a certain HUD element for your mod, feel free to make a request.
Or if you want to ask for some more information on how to create your own custom widgets, or maybe even help out others, or just ask some general questions - you can do that in this thread as well.

