So here's an idea. When a Knight gets to the vanguard rank, that Knight gets either one trinket or weapon slot opened or both, opened permanently .
It'd most likely just be a trink slot if any at all, which is still unlikely, but I don't see a major downside to it other than knights would be less likely to buy trinket slots after vanguard rank.
How about something like a permanent item in the supply depot for perma slots.