Global interwikis are an invaluable tool for wikifarms, but there isn't really any simple/consistent way to set such functionality up, let alone a sane interface for displaying and editing them once it is. Adding global support as an option to this extension, which already does the sane interface part, seems to me the best option to address this.

Basically I'm thinking it'd look like abusefilter in terms of configuration (central one has an iscentral global variable (unless it turns out there's no need for the central one to know it's any different from a non-global one), ones pulling from it have a global saying where they're pulling from, and if you're not doing anything global (in the cross-wiki sense) you just don't have either of these).

On the database end it'd basically just be merging the local database with the central/global database, with local interwikis overriding global (any interlanguages in the central/global table would be skipped). This would mean that sysadmins would probably want to delete most everything out of the local tables, since otherwise the redundancy would invalidate non-additive changes to the central/global table, but would also allow individual wikis to define their own interwikis which might not be relevant globally (besides interlanguages; lack of local interwikis is a significant limitation with InterwikiMagic, which otherwise patches this extension to work with a global table).

The list display on Special:Interwiki would be broken up into three tables: global, local, and languages. The global one is displayed for convenience but cannot be edited locally, and the local links are split into interwiki and interlanguage because it's a useful distinction to make, though both can be edited normally.

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