Let's take a new GM. New in the sense of "new to a specific game", not new to GMing. The GM read all the rules from cover to cover and he understands them, but never played. Should he start house ruling and/or creating new content before ever playing it?
closed as not constructive by mxyzplk♦ Mar 7 at 12:50
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There are two reasons for this. One is that reading the rules is very different from running the game. The other is that potential players also have a role to play in the creation of house rules.
If you don't have any direct experience of how things work at the table, it's a lot harder to feel where the balance actually lies. For example, creating a cool power that does a couple of extra things might look cool and balanced but turn out to take out far too much time to manage during play.
When it comes to house rules, the players you play with deserve to have their preferences reflected in those rules. They can't form an opinion on how the game plays if they don't experience it in raw form.
Equally so, when you approach a group that hasn't played a game before, you should play the game in raw form then show them the house rules you've developed in the past. That way, the group as a whole can develop it's own game.
It depends on the game.
Some games are designed like a building toy and have robust systems for creating new content. Others are designed as a holistic system with tight coupling between components. There's a spectrum of games between these two extremes.
Some games are written as a handbook on how and why to use the rules and systems presented. Others are a collection of rules and the reader is left to intuit the intentions and motives behind the design. Some of the former are right about how the game works, and others' self-analysis is flat-out wrong. Again, these things have a spectrum of games between the extremes of the two axes.
In this three-axis space, some combinations are going to be easy to create new rules and content for without ever having played. A robust building-toy design will need little to no experience to create functioning components using its built-in creation subsystems. What you build might not be elegant, but it will work. More familiarity with this sort of game will produce increasingly-elegant materials.
A holistic system that has clear and correct self-analysis can be easy to create things for, but you won't be able to tell that the analysis is useful and correct until you've actually played the game, and a holistic design that includes no explanations of its inner workings is a black box until you play it. Brewing materials for a holistic system before gaining an appreciation for its emergent behaviours risks unravelling its intended flow of play, preventing you from actually gaining any relevant experience of the way the system actually works.
And, of course, unless you are very experienced with system analysis yourself, being able to tell where a game falls on those three axes is itself prone to error.
Most games are not designed well, and don't do exactly what the designer intended. A few games are well-designed but only explain themselves enough to play. Even fewer are both well-designed and explained in enough detail to grasp without play experience.
The conclusion? If in any doubt at all, assume that you have to play the system for at least a while before you can understand its interactions well enough to homebrew or houserule for it. The games where this is not true are few and far between, and take some skill to recognise in the first place.
For house ruling that changes major mechanics, that depends on the experience that you want to have. There is nothing stopping someone from adding/changing rules before ever playing it; but doing so will mean that you don't get a chance to experience how the RAW is supposed to work in a group setting. Even with strong abilities in analysis, having different people with different ideas on how to play will put the system through it's paces (for good and for ill) better than a hypothetical understanding.
More minor changes, or changes you have seen discussed or recommended on the system will likely have less of a disruptive effect. Some systems even have house rules that more people have played with then without.
While jumping straight to house rules will make it more difficult to balance against expected outcomes in game given the GM in question's inexperience with it, it isn't impossible. Of course any opinions about the base system will have to be considered with the house rules that may have affected it, but there is nothing inherently wrong with utilizing what is essentially "Rule 0 with Structure", especially if you're willing to experiment and adjust as you go when things do or don't work. Sometimes something may not work in a way that gives you an idea for an even better house rule!
As for new content, I would use what exists as a strong guide and be ready to use the new content alongside the existing content so you have a baseline to compare with and balance against. So long as you are willing to be a bit flexible with your content should some aspects prove unworkable, new content should be no problem.