Whether a separate page is needed
editWe are here to write a generalist encyclopedia. While most experienced editors can appreciate that there is an element of activism in making heretofore unknown topics accessible as the first hit in a Google search, we subordinate those values to the priority of writing a certain kind of encyclopedia. This encyclopedia is free and value-neutral, written for a general audience (with jargon avoided or explained). As a tertiary source, we paraphrase reliable, secondary sources. Wikipedia puts verifiability before truth. The New York Times has editorial policy and fact-checkers, while we are faceless. Similar reliable, secondary sources are trusted to make assessments on fact. Their omissions are our omissions, as we cannot cover what isn't already reported by a reliable source. Accordingly, if a reliable source does not cover a fact, it is not worth including in the article. The content in each article is balanced in weight proportional to the depth in which our trusted sources cover the subject.
So we do not cover the minutiae of a game's cheats, fictional content, as that is (1) not what a general audience needs to know about the game, (2) usually not considered important enough to be covered in mainstream sources, and (3) better covered in other venues. Instead of asking how to justify a separate article, ask what sources exist on a topic and where they fit into existing articles. For an aspect of a game that is covered exclusively in game reviews, cover it in the game's article. For a small release in a series, cover it where the series is covered, whether in the developer's article, a game's Legacy section, or an existing article dedicated to the series. We follow summary style: cover a topic in a parent article until an overabundance of reliable, secondary sources warrant a split from the parent topic (lest the section take over the original article).
This deserves repeating: Topics only warrant separate Wikipedia articles when a overabundance of reliable, secondary sources cover the topic in compleat, such that we could do justice to the whole topic without resorting to primary or unreliable sources. If reliable sources cannot do the topic justice, we instead cover the topic in a parent article or not at all.
Completeness
editIn addition to our manual of style, the WikiProject Video games article guidelines generally describe the article components and related writing norms: how to format information, what types of information to include and exclude ... (maybe just edit the WPVG GL...)
https://www.merriam-webster.com/dictionary/opining
citation template, italicizing work/publisher