I've said multiple times before: DT2 members should require to be included by at least 2 different DT1 members (maybe even 3) in addition to having a net positive number of inclusions. This removes all the "soft" inclusions as well as removing the issue of people artificially inflating their own trust scores.

Linking default trust to signatures is a bad idea. It would encourage people only to aim for a default trust inclusion so they can monetize their account, and I have no doubt there would be some DT1 users who would include their friends/alt accounts/countrymen for the sole reason of joining signature campaigns, or even sell inclusions.