Retaining the feel of the suite, but based on newer technology
+
Improve the look with a new default theme
+ (based on toolkit's *stripe, using a new icon set)
+
Use toolkit's infrastructure where reasonably possible:
+ Extension Manager, help viewer,
+ update mechanism, satchel + new password manager,
+ toolbar customization, error console, etc.
+
Base suite-specific functionality and/or UI on toolkit where reasonable:
+ grippytoolbar as XBL extension to toolkit toolbars,
+ custom tabbrowser using toolkit's browser element,
+ several overlays to toolkit windows, etc.
+
Ease porting of Firefox/Thunderbird extensions to SeaMonkey
+
Make Lighting calendar work with SeaMonkey
+
Integrate a profile migrator (primary for old suites, FF/TB)