Ubuntu Software Center is supposed to replace several utilities provided by other piece of software as can be seen in the rationale, but I can't find equivalent functionality in Ubuntu Software Center. Where I can find the development plans of Ubuntu Software Center and any other packages included in Ubuntu?
For the general Ubuntu development you may want to check the Work Items Tracker. The amount included in this site is overwhelming since it covers all milestones on all aspects of Ubuntu.
The about page covers relatively what the site is: every aspect of development and bug targets of the meeting held by the different organs of Ubuntu, like Ubuntu Developer Submit, the MOTU meetings, and any other teams are captured, analyzed and tracked here.
Your specific concern is somewhere, about here. If it not, then you should try to bring the community attention to that.
You must ask the upstream developers of whichever package you are interested in, what their plans for the software is.
Ubuntu does not maintain a collection of roadmaps and plans for the individual pieces of software that are in the archive.