[[https://pinia.vuejs.org/|Pinia]] is the successor to Vuex. It promises to be lighter weight than Vuex (6KB for Pinia 2.0.13, 15KB for Vuex 4.0.2) and to support TypeScript better.
The main challenge we'd have to address in order to use Pinia is that Pinia is mostly compatible with Vuex 4, but not entirely. The API for creating a store is different (we could probably add a compatibility wrapper for this), and mutations don't exist in Pinia. Modifying code written for Vuex 4 such that it would work with both Vuex 4 and Pinia, or adding compatibility wrappers so that Vuex 4 code works with Pinia (which would be required for a smooth migration from Vuex to Pinia) may or may not be easy.
To install Pinia in MediaWiki, we would probably want to use `pinia.iife.prod.js` / `pinia.iife.js` and wrap them so that `VueDemi` is aliased to `require( 'vue' )`. Alternatively, we could use `pinia.prod.cjs`, but then we would have to set up a module alias for `vue-demi` to redirect to `vue`, and we would have to make `@vue/devtools-api` available (in the iife versions, that package is inlined).