I would like to be able to use incoming emails' headers, such as To (and I suppose From and CC as well) as trigger conditions. 

Use case:

I want tickets received via aliases to be assigned to the brand which is associated with the mailbox. We have, let's say, support@domain.com and service@domain.com, belonging to the same mailbox. The "support" alias is the primary one and is how the mailbox shows up in Channels, associated with Brand XY. Currently, tickets via the secondary alias "service" will be assigned to the default brand instead of to XY. Zendesk only offers a non-supported way with major caveats to deal with aliases to resolve this. issue.

Alternatively, Zendesk could make the entire headers available as a condition and offer a RegEx operator.