Self-Hosted Main

I am looking for an alternative to osTicket, which I currently use, and am hoping to call on the community's wisdom to steer me in the right direction. Please do share your recommendations and experiences. I was looking at Zammad quite seriously, but they do not have a system for collaborators that would work for me, and are resistant to the idea (rep kept refering to it as 'the collaborator mess', and ballpark pricing for developing an add-in seemed to be intended to make me go away).

A couple of requirements:

• Some of my clients rely heavily on the 'collaborators' functionality on osTicket. I would require the ability to have multiple users on a ticket.
• Some of my clients have office managers or IT departments/staff that would need 'manager level' access to see and respond to all of the tickets for thier company only.
• I would like a polished, client facing portal for user ticket submission and for users/managers to track tocket progress.
• Most of our tickets are currently submitted via email, so the ability to do so is a requirement, though I would prefer to move to a portal-type submission system, so tha ability to do that is required as well.
• I am hoping to find a white label solution.
• I am hoping to find an open source solution.
• I am hoping to find a solution that has a process for feature requests, and if possible, some system for funding particular feature development for things that I might need later.
• I am hoping to find something that is pretty to look at, since that seems to impress clients at least as much as function.