GusF wrote: [Tardis Index File is] very well established at this stage.
With whom? It's certainly not established very strongly in the minds of other Wikians. Go to w:c:community and you'll never hear "index file" there: it's always just "tardis".
Worse, Wikia Staff never call it that. So when we get a spotlight or other promotion within the Wikia network, we're always, always, always referred to as "Tardis". Look at the picture at right. The name given in the lower left-hand corner is not changeable by local admin. Staff thinks of us as "Tardis", so that's how we're portrayed in any Wikia-wide advertising. Other Wikians — that is, the ones who don't regularly contribute here — doubtlessly remember "Tardis" as our name, because we're one of the only wikis who can make links without the w:c: bit. If you want to link to iCarly, you have to type w:c:iCarly:iCarly. If you want to link to TARDIS, you only have to type tardis:TARDIS. That distinction is something that admin and active users right the way across Wikia notice and remember.
This proposal is thus merely bowing the reality of the power of our elegant, five-letter URL. It is not an attempt to practically change the name.