If you can believe some people, it often sounds like new content falls in one of two buckets: overpowered, or dead-on-arrival. While things are maybe not quite as extreme as that would imply, there is some truth to this. The new content could be more powerful than whatever is already there in the game, and it becomes a must-have for everybody. On the other side, if there is already an item that is more powerful, why would you go through all the effort to get it?
Of course, there is some nuance here. New content may be rare and/or expensive, meaning that for most players, it is out of reach. Even the second or third best alternative will remain useful. As time goes by though, items tend to become more common, and overall, even these most powerful items become more accessible. Even if we take away that layer though, it seems that items specifically fall in one of two categories: items that you must have to be at your most efficient, or items that... you don't.
This leads to a problem. You can't just keep making more and more powerful items. Especially in a game like RuneScape, with a hard level cap, there is a limit as to what you can do. Making a pickaxe that is better than the Pickaxe of Earth and Song, which requires the maximum level in the skill allowed, would immediately render all existing pickaxes obsolete. Introducing more powerful weapons or armour effectively drops the difficulty of all PvM across the board.
If you indeed believe in this one-dimensional thinking, you will get yourself stuck with coming up with new updates. What is often overlooked is the fact that there is a whole second dimension that we can play with. The best way to describe this "horizontal content" is as following: two or more items where there are valid use cases or circumstances for all of them. Let's break that down using an example in the recent Orthen digsite release: the Flow State relic power. This relic power gives you 20% more precision, but you will no longer receive soil. For many people, this may sound like a dream, but to others, it sounds like something they would never use. Screening soil is incredibly profitable, and especially as an ironman you may want any material you can get your hands on. If your goal is to get as many damaged artefacts as quickly as possible though, this is the relic power for you. Flow State therefore is a perfect example of horizontal content: it's not something everybody just defaults to, and adds a layer of choice.
There are two main approaches to making horizontal content. The first one we saw in our example above: a benefit that is paired with a cost. Another example of this approach can be found in AFKable training methods: you don't have to pay full attention (benefit), but you will often get much less xp/h and/or gh/h (cost). Whether a player wants to use this item will then come down to how much they value the benefit and the cost. This cost-benefit analysis may change depending on the circumstances, meaning a player may choose to use the item only for some things, and not others.
The second approach offsets the benefit not with a cost, but with a risk. This is often called risk-reward. A great example of this in RuneScape is the Dharok set effect (also the Berserker's Fury relic power): the lower your lifepoints are, the more damage you do. How close to death do you dare to balance to increase the damage you do? Different players have a different level of risk-aversity. Some players may be fine tackling a hard boss at 10% of their lifepoints to speed up their kills, but others - especially learners - would rather have the extra buffer.
I can name plenty of examples of horizontal content in RuneScape: different training methods, different combat styles (to some extent), different types of armour, etc. Still, I believe that horizontal content isn't used as much as it could be. I think the skilling offhands introduced in the latest Orthen Digsite update are an example of a big missed opportunity. The only "cost" involved in them are the cost of obtaining them, which is something that will fade over time. There is really no reason why you wouldn't want to use those skilling offhands all the time. Not only is that a form of power creep, but more importantly: it's boring. The skilling offhands provide bland, flat bonuses to your skilling, instead of adding a whole new dimension to choosing what to equip while skilling. Why is a twelve year old, almost forgotten piece of skilling content (the Inferno Adze) more mechanically interesting than something released in 2020?
There is a time and place for vertical content. There should be rewards across the entire level and skill range of the game. However, if you look at the game as a whole, it is incredibly fleshed out already. Combat has been flirting with the level cap for years, and several skills have their skilling tools maxed out at level 99. It is time to open our eyes to more interesting trade-offs. It is time to explore a new dimension of updates.