I am upgrading my electrical box with more circuits, and circuit breakers. I would like terminate all the unused circuits wires in the electrical box panel with dummy circuit breakers rather than leave them dangling in the box. I would then terminate the unused wires in the attic within an enclosed terminal block in the attic. When I need to add another circuit in the future, all I have to do is run the circuit from the terminal bock, and replace the dummy circuit breaker with a real one. This avoids the hassle of running a new wire into the panel from the attic, and crowding the panel with unorganized new wiring, etc. I rather do it all upfront. Is there any problems with this configuration other than code issues?
Cap off hot and neutral
The hot(s) and neutral wires should be capped off and laid in the panel somewhere out of the way. Don't attach even the neutral; neutral can be a live wire in certain edge conditions. You can attach ground, however.
How long to leave the wires? Long enough so every hot and neutral can reach every breaker space in the panel. This should be standard operating procedure for every circuit anyway*. Neutral is one of these because AFCI/GFCI.
* which is incompatible with the "Captain Snippy" method of making wires minimum length for an "ultra-neat panel" -- but you don't actually get brownie points for that. You get brownie points for a functional panel.