I posted this in this GSoC Project - but as Brecht pointed out, it’s way too much work for that - maybe at some point it can be picked up, so I just copy-paste it here:
The ability to create your own expressions
This could be a Node that allows text input and the creation of your own in- and outputs. You could then input for example:
outputA = inputA * pow(inputB, 2) + 1
This would of course be restricted to the existing Math operations. This way instead of creating huge networks of Math Nodes, a single Node with an expression would be enough.
Adding in and Outputs could work like it currently works with NodeGroups - in the Sidebar (with your additional functionality to controll the type).
An Example would be Rendermans PxrSeExpr