Changing the Help Center is not a simple thing — and it shouldn't be. Most of it is in the control of Stack Exchange, not the individual Stacks. This is a perfectly good place to create exactly what you want. I'll convert this answer to a community wiki so that anyone and everyone can add to it.
Contributors: Please alphabetize contributions. Thank you.
Worldbuilding.SE Abbreviations, Terms, and Slang
Note that there is a Stack Exchange Glossary - Dictionary of Commonly-Used Terms which may also be useful to consult.
Frame-challenge - Challenges the question's approach to a problem, re-frames the problem, then solvs the newly framed problem. (Can require considerable persuasion to pull-off succesfully, see answer here or here.) A meta-discussion introducing the concept of frame challenges to new users is found here.
Handwave - Purposely glossing over a known logical discrepancy in something. A handwave is often an intentional decision to ignore one issue, to instead focus one's energy on developing something more interesting.
Handwavium - The mechanical equivalent of Unobtanium. Handwavium is a device or technology that "just exists" within the scope of a question/answer, how it works being irrelevant. See here.
Off-Topic - A phrase that indicates the question is considered inappropriate or in violation of the rules and intent of the site. For more information see here.
OP - Original Poster - An acronym that identifies the author of the matter being discussed. In the majority of cases, it refers to the author of the question. However, in comments to an answer, it can (uncommonly) refer to the author of the answer.
Opinion-Based - Refers to both the Stack Exchange cause for question closure and the Worldbuilding-specific meaning. In short: either it is believed no answer can be given to a question without basing the answer on opinion or the OP cannot judge a best answer in any way other than by opinion. For more information click here.
Phlebotinum - A less-used synonym of Handwavium. Most commonly seen in the phrase applied phlebotinum.
SE - Stack Exchange - This abbreviation refers to the Stack Exchange/Stack Overflow corporation and/or the general policies of Stack Exchange. In most cases this refers to the site Help Center but can also refer to policies and discussions on Meta.SE.com.
Stack(s) - A community within Stack Exchange.
TL;DR - Too Long... Didn't Read - Used to indicate the end of background or supporting information and the beginning of the summary or gist of the post.
Unobtanium - A (fictitious) chemical, alloy, or substance which perfectly fulfills the requirements for a usage case. The term is famously used in James Cameron's Avatar. Another classic example is Vibranium from the MCU. See here.
VTC - Vote to Close - An acronym that usually indicates the comment is explaining a reason a close vote was cast. Users are not obligated to provide explanations and the use of this acronym does not imply they should.
VTC:OT-NAW - Vote to Close: Off-Topic - Not About Worldbuilding - See here.
VTC:OT-SITE - Vote to Close: Off-Topic - Question belongs on another site in the SE network - See here.
VTC:POB - Vote to Close: Primarily Opinion-Based - See here.
VTR - Vote to Reopen - An acronym that usually indicates the comment is explaining a reason a close reopen was cast or an explanation of what could be done to secure a vote to reopen. Users are not obligated to provide explanations and the use of this acronym does not imply they should.
WB.SE - Worldbuilding.SE - This abbreviation refers to this sub-site of Stack Exchange. Like SE, often invoked when discussing policies.