Some potential gadgets, based on a conversation with @SGill:

• CharInsert (standardization of markup options at top)
• Formatting buttons, such as "center" (sometimes, each button is a specific gadget; other times, buttons are generated via script)
• Clean Up (found under 'TemplateScript --> this is a script, not a gadget), which helps clean up formatting
• Easy LST (you don't have to click on edit/next repeatedly).

Some more potential gadgets, mentioned by @Samwilson:

• mark
• no-op inserter

Note that the CharInsert gadget (e.g. https://en.wikisource.org/wiki/MediaWiki:Gadget-charinsert-core.js ) is a bit confusing: there is a CharInsert extension already installed, but it's insert definitions (which live in the edittools system message) are no longer used, and instead the gadget adds its own. I'm not really familiar with all the history here, but it seems like there have been various ideas to make it easier for wikis to define their own lists of insertable snippets (including per-user). There's also a bit of overlap with the 'special characters' bit of WikiEditor. It sounds like T210561 might be a good one for us to tackle for this (although it's not Wikisource-specific).