Plane Sailing
Astral Admin - Mwahahaha!
Another possibility would be to remove 'Mark' entirely, and replace it with 'Taunt'.
Taunt
Minor action. You taunt an opponent you could reach with your weapon. [optionally: make a Cha vs Will attack and if successful], the opponent must include you in his attacks next turn.
That would give a flavourful minor action activity for the fighters, doesn't give the fighters the opportunity to pump up their damage output via extra attacks on people shifting or hitting other guys but certainly helps in the defendery role by 'attracting aggro' as it were.
Cheers
Well, I discussed this with my players last night, they liked the idea and it worked really well in play for the two fighters.
Taunt
Minor action. You taunt an opponent you could reach with your weapon. The opponent must include you in his attacks next turn.
So no penalties to hit for anyone, no free strikes against people shifting or attacking someone else, but the taunted target is forced to include the fighter who most recently taunted him in his attacks.
This worked really nicely on a mechanical level for us, and it was much more flavourful as well - the fighters enjoyed using their minor actions to taunt people and keep the bad guys attention focussed on them.
Cheers