Tough goals,
I'm looking at this Project (the old lady) very closely and have to say, that I was very impressed, of what I've seen (and of course worked with).
As for the featurelist I think that it sounds hard to complete within a short period of time, but I hope you'll make as perfect as MP I.
One thing to remember though is that if you "allow" the plugin designers to change and modify code close to the core of MP II it might be difficult to make sure, that the endusers don't get confused about what plugin designers changed and what dependencies result in this.
And as you told about the change of the "audience" of the MP users. When you look at the market of HTPC's you'll see that the devices get smaller and smaller. So if you want to use your system within your livingroom, they have to be as silent as possible. But this directly interfere with the need of "highperformance" systems and gfx-cards. So one goal should be to make the system as attractive as possible for chipset-gfx-based Mainboards.
The last thing to mention is that you want to allow to change the configuration within MP II. Please consider then to find a way to "block" this feature by the external configuration. When I think of my wife ("I did nothing dear. It broke by itself")
, I see lots of work for fixing to come...
Beside that I'm waiting to see the new version.