My suggestion is to add an optional expandable text input pane where any string could be written. This pane would be visible when choosing C# Program (or VB / F# Program) in the language drop down. The program template would then feature a Main(string arg) where the input would be fed.

Many times I find myself pasting json, xml, csv, tsv data etc into the programming pane of linqpad. Afterwards I have to put the text into a string variable and replace quoute characters and other special chars to make it compile. Sometimes I instead write the text into a file and load the file into the variable. Both solutions work and are not to hard to acheive, however it would be quicker just to paste the text into a textbox and have it readily available in the program for processing.

For me this would be a productivity feature I would use quite a lot.