Alt thieves are already immune to justice because, well, they're throwaway alts. If they get caught and killed there's no loss because that was it's purpose to serve. It's the alt's neck instead of the player's main. While thieves should be able to make a living as a thief without immediately getting the axe, making it so they're more difficult to hunt down won't change the market for faceless suicide thieves. Some players like to play outside the means given or just want to covertly be dicks, and the best way to do that no matter what will be creating an entirely unrelated character and using him to steal and grief while your main assets are immune to retribution.
I think it would be neat if the dev's bloodline idea created clans and families for the players where alts are just different relatives of the same bloodline. Alt thieves getting tracked back to a clan and having the clan face justice. The purpose here would be to tier skills in a way that things like the black arts are only available to clans of certain sizes and experience, so that a person developing a clan would have legitimate access to the black arts, making it difficult to just clogspam your way to a throwaway alt thief. Of course, there's no way to make players stick to just the one family and then it will shift from alt character thieves to alt bloodline thief clans.
Serve justice by chasing the guilty clan barefoot with an armed mob of prosecutors, through forests and across rivers. Survivors are clearly innocent.