In your proposal I am missing information about when you propose to tackle the dynamic features (hair and cloth) and the material nodes. I do assume you do not intent to handle this in the first stage but you will need to address these issues. Furthermore a lot of people would like to know if you would be able to handle collisions in a friendly way and things like fluid and soft body dynamics. These are areas where Poser is very weak.

I understand you don't intend to develop a fully fledged modeler and I do expect that such a modeler will probably be available quite soon in the project as a plug-in (several are available so it is likely that someone would integrate an existing one) but should provide a capability to create morphs and design clothing as soon as possible as these appear to be common wishes of all initial users.

Personally I would not spend effort in developing a renderer in the early stages but instead use a renderer already available. Any developed renderer should always be just another available renderer as you should strive to support as many available renderers as possible (including whatever material setup they require). It will be important to understand and correctly handle the materials (with all nodes) used in both Poser and DAZ Studio and to convert these to the materials as appropriate for the different renderers (this will likely be one of the major challenges). In the first stages only support for the most basic materials is required.
Animation is a nice to have but most people using Poser have no need for animations as they mostly produce stills only. Currently within Poser it is required to set up an animation for the calculation of dynamics, I would prefer to remove this requirement as it is unclear to many users and would propose to handle this in a modifiable draping (which is in effect an animation but can be provided in an easier way).

Cloning of object is also important and should happen automatically, when another copy of an object is added it should become a clone. The system should also be intelligent enough that common materials should actually be shared unless specifically disconnected. This means that if you add a chair to your scene and than add another of the same chair, they be become cloned objects, if you then add a table using the same basic wood material, the materials become shared, so changing the wood will automatically change both chairs and the table.

When defining the GUI make sure you include the dials as they currently exist in Poser (although people complain about them, they equally complain about other applications not having them) together with some other options like numeric entries only and sliders, for angles a clock face with a line would be convenient. Please allow model developers to provide a capability to hide specific variables and to allow only specif values or value ranges for specific variables.

In all a good start.