Since themes (surveys & questions) are part of the game, I would suggest "add-ons". But when you think about finding a suitable term in other languags "extensions" might be better.
If I get the intention of the shop right, the add-ons should be only offered inside of the LimeSurvey application. The store should not simple replace the function of the Joomla file repository (LimeSurvey.org -> Downloads).
If the user should be able to use add-ons without manual downloading and installing them, the way LimeSurvey and add-ons are offering updates gets a lot more critical.
More users will install more add-ons. Will e.g. ComfortUpdate analyze these add-ons? Will ComfortUpdate deactivate all add-ons during update? Or will ComfortUpdate access all add-ons for version compatibility upfront and demand update of add-ons first? The release early, release often approach in combination with an add-on shop will demand some change to get along smoothly.
Will there be a system to select the correct versions of add-ons? Upgrade/Downgrade of plug-ins? Or themes?
Long term aim: Packaging?
research.swtch.com/vgo-mvs
/
blog.golang.org/versioning-proposal
Currently a LimeSurvey user has to have some knowledge to choose, download and install a plug-in. If a plug-in breaks e.g. LimeSurvey login, you could be sure that the user had access to the file-system. Removing plug-ins from the folder would be the current quick fix.
The theme and plug-in developers might have some questions about offering items too. Will the addon-shop be more like Wordpress (SaaS via Wordpress and external payment when selfhosted) or more like Apple App-Store (Payment via Store only). Who will handle the payment is a interesting part. Add-ons could be offered via a freemium business model and a key could be sold to upgrade a add-on. Personally I would like to see a central payment and product bundles to get transactions easier handled. Like Themeforest has done to the Wordpress theme market. But I have no idea how many developers are interested in the distribution via a central shop.