Make XAML a first class citizen of Visual Studio
Make XAML participate in compile time syntax checking, in code refactoring (e.g. property renames), and in code navigation. Add refactoring (e.g. "Create Style from Element") and other performance features to the XAML code editor (e.g. allow in-place editing of xaml tags so that the end tag changes when you change the begin tag).
And able to fully contain other first class ciztizens like word, excel, outlook.. etc.
Mike Eshva commented
Eduardo Molteni commented
And don't forget to allow FixedDocuments and FlowDocuments to be editable in the designer
I'd highly recommend using a combination of ReSharper and "XAML Power Toys".
Joel Barsotti commented
Being able to refactor the XAML would be HUGE!
Highlight a reference in code-editor for named tags...
FYI, ReSharper provides several improvements to the Xaml designer in both VS2008 and VS2010. It also makes the appropriate changes to Xaml files when you run ReSharper's refactorings. It's a great tool for .NET development in general, doubly so if you're a WPF developer. It provides great Xaml IntelliSense too, which works with markup extensions and bindings (including property path syntax). Although VS2010 already includes vastly improved Xaml IntelliSense, ReSharper provides all the same functionality and more for VS2008 too.
Intellisense for markup extensions and bindings would be cool too.
Kendrick Hang commented
I'll agree with @jdu. "Make XAML a first class citizen in Visual Studio" is a broad request, but the parts I would put priority on are getting the "find all references" and the "rename a property" features to work with XAML. Non-trivial features to implement, but so essential for WPF application development (and maintenance).
Collapsible regions in XAML would also be very useful.
The #1 point for me is that when I make code analysis (e.g. "Find all references") or refactoring (e.g. rename a property), I should be sure with 100% confidence that I'm correct. Otherwise it's just an invitation to create new bugs.
Today I think some of this works (renaming a method, which is bound to an event in XAML), but some doesn't (e.g. Bindings to properties). I don't know how the XAML editor could be smart enough to know which type of object the property is bound to, though.
"Create style from Element" I love that one!