I develop a lot of extensions for Fiddler. Some of them are public, others are private to my organization. Currently Fiddler4 is built against the .NET Framework 4.0. I don't want my extensions to depend on a different version of the .NET Framework than what Fiddler requires, which lowers the burden of distributing them. If Fiddler runs, then I can be sure my 4.0 extension will too.
It would be handy if Fiddler updated the .NET Framework version requirement. It would 1) give you guys more Framework to work with, and 2) allow me to up the framework requirement of my extension while still meeting the bare minimum that Fiddler itself requires.
There is a considerable amount of new cryptographic faculties in the .NET Framework 4.6.1 that would be handy for my uses.
You can safely develop extensions targeting .NET 4.0. Less than 5% of Fiddler users would use an earlier framework, so we'll deprecate the 2.0 build soon. Don't go above 4.0, though since a significant portion of Fiddler users stick with 4.0.
We are actually currently in the process of doing that. Rather unfortunately, it is going to take a couple of months before we do that since we need to collect more statistical data on the .NET frameworks Fiddler user have installed. We'd like to update the framework version as much as possible without leaving any significant portion of Fiddler users behind.
So, please, keep an eye on the status of this task here.