My take on it (from earlier post), for what it's worth:
Just a tweak for the Don will be fine.
Normally I would say make him invisible to investigations but if hitman gets killed early they're screwed, and if thief dies no more bullets, so I think that's the biggest risk. I think the Don could be made invisible to investigations while he has no additional powers (except for his recruitment power), and once he acquires powers his invisibility to investigations disappears.
Once a mafia member dies the recruit or Don can take over their role but at 50% efficiency (for both thief and hitman roles, but if hitman misses he doesn't waste a bullet). If a second member dies the Don can take over that role too with the same limitations, however the recruit can only have one role so he is whoever he chose that first time someone died (same as the last game).
Example:
Mafia team at start: Don, Hitman, Thief.
Don recruits: Don, Hitman, Thief, Recruit (no powers).
Thief gets killed: Don, Hitman, Recruit (Thief powers at 50%). (makes no sense for Don to take Thief powers as he would lose investigative invisibility)
Recruit gets killed: Don (Thief powers at 50%), Hitman . (Don is now visible to investigations)
Hitman dies: Don (Hitman powers at 50%, Thief powers at 50%). (Don can choose to be a hitman OR a thief each night, can't do both).
In the previous example when it was just the Don & the Hitman, if the Don was killed next then the Hitman would be Hitman only as he can't become the thief. Only the recruit can take on another role (one time only when first mafia dies, same as the last game), or the Don can (multiple times, whenever a mafia role dies).