I see why it would be boring, given its popularity, shortness and the fact that it’s a nickname for the most common name ever, [name_m]John[/name_m]. But I think [name_m]Jack[/name_m] can make up for that with its cool, energetic sound.
[name_f][/name_f]I also wouldn’t combine [name_m]Jack[/name_m] [name_m]Max[/name_m], they give a similar feeling but sound very choppy when said together. They’d make good brothers though! You could go with [name_m]Jack[/name_m] [name_m]Maximilian[/name_m], [name_m]Jack[/name_m] [name_m]Maximus[/name_m] or [name_m]Jack[/name_m] Maxwell… or just use [name_m]Jack[/name_m] [name_m]Max[/name_m], honestly. Bad flow isn’t the end of the world.
[name_f][/name_f]If you still think [name_m]Jack[/name_m] is to boring, there is a plethora of long forms that could lead to [name_m]Jack[/name_m] as a nickname! Some great suggestions in this thread already.