Wiktionary Mobile
I suspect not, as quite a lot of scripts depend on the current format. (VisibilityToggles would also be completely unusable if we switched to mw-collapsible.) I'm not sure this would really be a real solution in any case. Making Wiktionary usable on a mobile device without having any possibility of site-specific CSS/JS doesn't seem all that feasible. Most lists on Wiktionary are worked into multiple columns when they are large enough, and almost all expandable tables are multi-column, which doesn't fit on a small device's screen. The French Wiktionary also needs a solution for translation tables, and their tables are all either not collapsible or just not collapsed by default, so if they switched their collapsible tables to use mw-collapsible, it wouldn't be a solution for mobile. If the mobile application is to allow editing (not directly through wikitext), or provide an equivalent of targeted translations, site-specific JS will be required.