A very simple node showing a particle node concept test. This is just one possible way of doing this (and not a very good one). But since it's the first time particles can be modified by nodes, i wanted to show it anyway.

The node called "Newtonian" here (misleading name, just kept it because changing it is tedious and pointless) simply moves particles along +x in each step. This is done with a callback function handed over to the "Program" node with the "Exec"/"Port" socket.

As shown here, these nodes can be used multiple times and in arbitrary order, thereby multiplying the effect. Later on, such execution sockets could be nested, so e.g. emitters can have an execution socket to call operators on the freshly emitted particles to define their initial state.