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.
You need the correct parts
The main Code issue with your plan is that your enclosed terminal block needs to be UL listed as an assembly in order to be acceptable to the AHJ; since terminal blocks are UL recognized components, they are Rather Useless to us by themselves. Fortunately, there are enclosure-mounted terminal blocks that meet the above constraint available through a couple of vendors; the most useful one for your application is a McMaster-Carr 3691T14, as it provides room for 10 hots and their corresponding neutrals, and can handle a mix of 15A and 20A branch circuits. You'll have to holesaw some 1/2" trade size (7/8" actual size) knockouts in the sides for NM cable clamps to get the cables in and out of this box, use some knockout plugs to plug the KOs in the ends, and squeeze a pair of Penn-Union NA-412 ground bars, linked by a piece of 10AWG bare copper wire and mounted to the box using 1/4-28 self-tapping screws, into the box to land the equipment grounding conductors to.
For 30A and larger circuits, if you wish to carry this approach to larger sizes, your best bet is to get a bog-standard NEMA 1 pull box from your local electrical supply house and mount some UL listed, 14-4AWG, power distribution (splicer/reducer) blocks in it; while they take up a fair bit of space (3/4" to 1" of width per pole), you won't need nearly as many poles here as you would if you tried this for every circuit under the sun, so the box size can stay semi-reasonable here. You'll need to whack another ground bar in this box, too; for a reasonable circuit count, another Penn-Union NA-412 will do the trick, or if you need something larger, you can use its bigger brother, the NA-413. Again, these mount to the enclosure with 1/4-28 self-tapping screws; you'll also need screws to mount the power distribution blocks (or the DIN rail they're attached to) to the enclosure, although these can be ordinary sheet-metal screws.