Possible features of Silverlight 5:

1- More Platform Reach

When talking about “reach” there are a few interesting dimensions:

a) Silverlight Browser Support: Silverlight currently only officially supports Internet Explorer, FireFox, Chrome and Safari. Support could be extended to Opera or other less prominent browsers.

b) Operating System: Silverlight is currently limited in the Mobile space (No Symbian support yet) and in the Gaming Console space.

c) CPU Architecture: Silverlight doesn’t support x64 CPUs, Mac G4 PowerBook CPUs, and a few other CPU architectures.

2- Bundle Silverlight with Windows

At Mix10 Microsoft VP Scott Guthrie announced Silverlight Adoption rate is at 60% of all internet connected machines.
In the future, we could hope to see Microsoft bundle Silverlight with Windows or add it as a super-duper-should-have update to Windows Machines.

3- Improved WriteableBitmap API

The in-memory drawing API in Silverlight is somewhat limited for advanced tasks, like:

1) No API for Drawing Shapes.
2) No API for Resizing & Rescaling.
3) No API for Clear, Copy & Crop methods.
4) No API for Conversion to various formats.
5) Performance issues that plague any “1 pixel, 1 read/write” approach.

4- 3D support

5- Store File Permissions Granted by the User

Using the OpenFileDialog and SaveFileDialog end-users can respectively grant read and write permissions for any file to a Silverlight Application.
Developers would like to see these permissions persist after the application has been restarted. We can see in Silverlight 4 RTM that Full Screen, Webcam and Clipboard access already get persisted after application restart.
So it is not unreasonable to hope this feature will be included in Silverlight.

6- Add PLINQ and TPL support

With PLINQ & TPL creating, debugging and maintaining multi core applications has been massively simplified on the .Net framework. With Silverlight supporting C# 4 and a majority on new .Net 4.0 features, it is possible we’ll see PLINQ and TPL in future Silverlight versions. That would allow Silverlight to have the best in-browser threading  solution for any major programming platform.

7- Reporting Controls and SQL Reporting Services Integration

Microsoft has a great suite of Reporting and Analysis tools that are part of the SQL Server 2008 product group. As part of ASP.Net and winforms there are ReportViewer controls that allow to view, zoom, page and export from predefined RDLS reports. If this feature is implemented it would probably include a new Silverlight & WPF Report Viewer control as well as deep SQL Server Reporting Services integration.