If the number of items the player can have increases over time, this means that each individual item's significance decreases over time. This has the effect of making decisions easier over the course of the game.

This is only true if the resource cost of things remains a constant across the game's lifetime. Most (if not all) games tend to increase costs as time goes on.

There's also the consideration of what a player will do with a larger stack of resources. You might incentivize them to e.g. run two factories as opposed to one, therefore naturally increasing their resource usage while also doubling their gameplay.

If the items get more powerful over time, then items acquired near the start of the game have no viability later on.

This is hard to answer without specifics on your game, but there are cases where new resources supplant older resources, and cases where new resources simply add more options. If the new resources are exceedingly rarer, or they have their own set of drawbacks, then the player is urged to still use the older resources where possible, just so they don't overuse their rarer resources.

For example, crafting a dagger is the same whether you're using stone, iron, gold, or veryexpensivium. But these later resources are only found later in the game, and rarer. The player has the option to hunt for rare veryexpensivium and only craft veryexpensivium daggers to make money, or to just easily collect stone and make a ton of stone daggers.

Similarly, while a Greater Health Potion is obviously better than a Lesser Health Potion, it's significantly more expensive to craft and it has a much longer cooldown period for usage. Therefore, players are advised to still use LHPs when they only need to heal a little bit.

Simply increasing the variety of items over the course of the game is a sure-fire way to lose players early if they believe they are not making progress.

I don't quite follow. How would a progression of resource acquisition make the players believe they are not progressing? Quite the opposite, no?

This is where vertical and horizontal progression start to become relevant considerations:

• Horizontal progression means variety. A shotgun is different from an assault rifle is different from a sniper rifle, but they all have pros and cons. Overall, they average out at the same quality.
• Vertical progression means improvement. A laser shotgun does more damage than a regular shotgun. It may be harder to come by, but if you have it, it's the superior choice.
  • Horizontally, a laser shotgun is different from a laser assault rifle is different from a laser sniper rifle, but they all have pros and cons. Overall, they average out at the same quality.

Not all players are created equally. Some like horizontal progression more (so they are on par with experience players in terms of equipment, just not player skill), others like vertical progression more (so they feel like they are improving their equipment and becoming more powerful).

Which kind of progression you implement highly depends on the kind of game you have, and how you expect players to interact.

• In PVP games, vertical progression deincentivizes new players, as they cannot beat experienced players. At best, you can make "ranks" so experts do not play against newcomers.
• In single-player (or single-player-focused) games centered around expansion, vertical progression incentivizes a player to keep player, as they increase their value and amass wealth/power.
• In games centered around short-term gameplay (think: roguelikes, sports games, fighters), vertical progression is not as important as horizontal progression. There's usually a bit of both, but predominantly more horizontal progression. I.e. you unlock new fighters rather than your fighter becoming stronger than the other available fighters.

Vertical progression is generally more incentivizing in terms of single player content, but it clashes with multiplayer content when dealing with players who started at a different time but wish to play against each other.

Horizontal progression doesn't incentivize over the long term, but it does create a replayability and creativity factor that works well for short games that are meant to be played often (as opposed to a single game that is played for a long stretch).

Overgeneralized:

• Vertical => Singleplayer, one long savegame.
• Horizontal => Multiplayer or short games with high replayability.

Is there a way to limit players other than a currency system or a resource system?

You've glossed over time. Now, to be clear here, I'm not referring to "microtransaction energy" systems specifically, but I am referring to a time lock. Essentially, a cooldown system.

But the main takeaway here is that it's not how you limit the player that matters. What matters is how the player can work with those limits.

• Maybe your smith can only craft one item per day for you.
• Maybe you have infinite potions, but they are toxic and you can't chug them all (cfr the Witcher).
• Maybe you have infinite ammo in your bag, but reloading takes a long time.
• Maybe you have infinite ammo in your gun, but it overheats.

All of these limits need a way to massage them. Not for players to outright bypass them, but to adjust the limit based on some actions they take.

• Paying your smith +50% for each job makes him happier to do two jobs a day.
• There's a clearing potion that lowers toxicity, sort of like coating your stomach, possible at the cost of a decreased potion effected.
• Upgrade your gun to improve loading speed (possible at the cost of e.g. smaller clips)
• The overheating mechanic could be impacted by local weather (tropical vs arctic), or what metal the gun was made from.

The best games present you with both an obstacle, and tools to (partially) overcome that obstacle, possibly at the cost of effort to set it up. The worst games set up arbitrary limits that have no in-universe justification and create no gameplay around dealing with that limit.