Most likely, we will have it. It is one of the biggest features of the original architecture.
BUT !! I can't speak for everyone on the team. W3bbo is the project lead and will be programming the crux of the new architecture. Many others will be able to add packs (again, nothing's set in stone here) and have swappable themes and styles into it.
I am not completely sure how the new architecture (built on C#) will translate into slipstreaming. But, if it isn't in the first new 5.0 (alpha/ beta), we might/ will be looking to add it into the future.
First is first:: Get the program set, architecture written, and have it's ability to patch the OS perfect to near 4.x effect or greater. After, we will add features and stuff that will make XPize's/ Vize's capabilities expanded.
Right now, just be patient. The program will be worked on, no doubt about that