Two points I would find very helpfull to add to the Tracker-Thunderbird addon:

1) index the complete address book of Thunderbird to get all contacts:
If you use Thunderbird as a PIM and administer your address book this will probably a lot of valueable information which can get connected by tracker with information from other applications with userdata (facebook, Pidgin, flickr, creator in office docs, ...)

There is a -- in my eyes -- very usefull addon for thunderbirds address book, called "MoreFunctionsForAddressBook"[1]
This addon mainly gives two advantages which could be usefull for Tracker:
- add more fields to address book item & export address book item as vcf-Card: advantage here is, that I can specify the usage of the additional fields in the vcf-export. So I have used the additional fields to store information like Jabber/ICQ/AIM/MSN/YAHOO/... nicks and used the corresponding csv-values as export options. It would be very nice if Tracker could also read this data. It would make it probabbly much easier to connect later on chat-history from e.g. Pidgin, mails and address book information from Thunderbird to the right/same contact.
- support for Tags: I don't know the ontology used by Tracker, but it would be nice to find the contacts by the assigned tags and to write back tags to the address book items that were assigned in Tracker
(as far as I know the fields won't get deleted if the addon gets deleted, so there may be a chance to be independet from "MoreFunctionsForAddressBook" in the Tracker-Thunderbird addon)

2) try to link from/to/cc/bcc of the indexed mails to the corresponding contact out of the address book if there is a address book item with a matching email address:
The link itself of the seperated information email and address book item will add a lot of usefull information to the data



[1] http://freeshell.de/~kaosmos/morecols-en.html
[2] http://en.wikipedia.org/wiki/VCard#vCard_extensions