Short version 😀

During the period where a space empire will be dependent on fusion energy this three elements will be vital to its growth and prosperity.

Long version 😯

Deuterium (2H or D) is an Hydrogen atom with an extra neutron in his nucleus, it is a stable element and it has been probably produced in the today measurable quantities in the Big Bang.

Deuterium is consumed in the fusion activities in the stars and is rarely created by natural process.

Its percentage in respect to the total Hydrogen is around the 0.015.

Deuterium is an extremely useful material for various activities even before the Fusion energy era: water with a big percentage of deuterium atoms is called Heavy water and is extremely useful in fission reactors and other medical and research processes.

In the fusion era Deuterium, with his friends Tritium and Helium 3 will be the most efficient (military grade) fusion reactors fuel.

Tritium (3H or T) is an Hydrogen atom with two extra neutron in his nucleus, it is extremely rare in nature and is radioactive (no- dangerously) with an half-life of around thirteen years (its decays into helium 3).

It can be produced with various, not really efficient atomic reactions and in fission reactors when deuterium (rarely) captures a neutron.

Tritium is already used in nuclear and thermonuclear weapons and it will be very useful in the fusion age.

Helium 3 is an extremely rare non-radioactive isotope of helium with two protons and just one neutron.

It is another quite useful fusion fuel but its extraction on Earth is economically expansive and probably not worth the effort.

Giant planet's atmosphere mining can potentially be much more interesting and, for an initial jump-start of the fusion age, also moon regolith mining can be a good if less efficient source.

These three elements can fuel an entire game age and places with rich concentrations can be strategic and war-worthy locations.

Disclaimer 😳

When I talk about 'Spice' I'm not proposing to add it to a game, it is just one of the most famous 'luxuries' in SciFi literature and I'm taking/proposing inspiration from.

Short version 😀

Spice was an interesting 'Luxury' in the sense that it had, aside from the pleasurable effects, also important uses: Starship pilots had to use it regularly to be able to do their job.

Long version 😯

One thing that I see over and over again in 4X games is that theoretically important part of the game like luxuries, technologies, government types, etc. becomes banal and commoditized after a few games.

Luxuries are rarely very different from one to the other aside maybe some effects that after a while are thrown back in the mental created luxury back box and then just use is like that, a box that you can exchange with other boxes and do your boxes collection.

Another problems stems from the inevitable simplification that is plaguing this kind of games: the idea that the government are driving the states and the economies.

This is plainly not true in the reality and that is strongly visible in the difference between what is really happening and how the games are working.

Unfortunately as simplifications goes this one is quite difficult to avoid without plunging in a maelstrom of complications.

So, what can be done have better and more interesting luxuries?

As the Spice example tells us the luxuries need to be more than a fancy name and a few bonus/malus parameters.

Luxuries should be part of the Setting and possibly the storyline. Their discovery should be momentous, possibly an event in itself. The reason behind the luxury status of a resource should be clear and, maybe, even variable and evolving with the storyline.

Luxuries could also have ties with the game system: like the Dune 'Spice' that was indispensable to precisely navigate the FTL starships of that setting.

That will make them more difficult to invent but much more meaningful.

There should be specific investment and needs linked to them, like specific farms or mines and there should be a market, a unique building in the known space where the majority of the exchanges about that luxury is made.

It is like the White Truffles: the market in the Italian city of Alba (BTW my grandmother city) is The place where to go to buy it in big quantities and that brings a lot of bonus to the area (BTW they make also really good wine!).

Luxuries are important, history-wise they are the prime motor of commerce and commerce is the primary motor for richness that is the primary motor for anything else.

The choices presented to the player about luxuries should be meaningful, fun and articulated. Just a standard strategy that always work and that is the same for them all will not cut it.

Short version 😀

In the space age, sub-atomic mechanics science will have advanced far enough, that we can craft virtually any resource from any other resource. So the main measurements of resources would be mass and energy.

Long version 😯

In order to provide for the economy, massive amounts of mass (any material) would have to be mined and transformed into other materials. These conversions would require humungous amounts of energy, requiring dyson-sphere like contraptions in order to satisfy the hunger for energy. Eventually, later on, it should be possible to convert pure energy into materials, so even mining entire planets away ( 😈 ) would become useless. So my proposal is mass and energy. 😎

Normal mass and normal energy?

What about dark matter and dark energy? 🙂

Jokes aside thanks for the nice contribution!

What I can read behind the lines of your post is that at a certain tech level the usual material we use now will become unlimitedly available.

When pico-tech will be mature we will be able to convert atoms at will and even, to a certain extent, create new ones never been naturally available. Quantum dots also will let us to create designer atoms with funky characteristics.

When we will able to tap into dark energy (actually credited as the biggest part of our universe) or to sip energy from other universes that also will become moot as a resource (and by consequence matter also).

What will be left?

Magnetic monopole? Theoretically needed if this universe birth was really a Big bang but extremely rare.

Information? Quality information can be rare as magnetic monopole: just look at the TV news!

Memes?

Tickets for Sublimation?

The starting point of your post is nice but I would put it in the later period of a 4X game .

That is the point: resources will be different depending on which age of the game is behind played. Deuterium and Tritium are good starting resources, normal matter and normal energy are interesting 3/4 of the game resources.

Let's find out other ones to fill up the slots for the various game ages.

Deuterium, Tritium and Helium-3 would be good starting resources, agreed.

I think the main problem is the phase between this and the thing I suggested. I can't really think of what would be used, besides standard materials. Possibly that any material is used to generate energy through mass-annihilation or singularity power generation.