An off-action close burst attack would be nice imo, but I can't find any I can use. I'm a fighter/multiclass rogue.
There are a few good strategies to optimal use of this power, and some of them will depend on your friends, some of them you can pull off just by yourself.
Let's start with the bad news though: There isn't a power on the rogue or fighter list that is close burst 1 and can be done as a minor action. Let's look at the options that are available:
Tact 1: more damage on your turn. Pick a burst 1 encounter attack power (Sweeping Blow is the go to for fighters at low levels), and use your action point to fire it off.
Tact 2: status effects. Pick a utility or attack power that is burst 1 or 2 and inflicts something nasty. Perfect Feint, Glowering Threat, or with the Arcane Trickster path, Blinding Flare.
Tact 3: out of turn damage. There are several fighter stances that let you attack or otherwise deal damage out of turn. My fighter uses Rain of Steel and Bloody Blades as two powers that accomplish this (both are dailies, Rain of Steel is on the fighter list, Bloody Blades is a power swap from the Gladiator theme). The other element of this is to teleport away (classic choice would be being an Eladrin) and then let your buddies drop area affects on the enemies you've gathered.
One last thing on the out of turn actions, and it's a bit on the homebrewish side, would be to work with your DM to sub out your marking for the Knight's defender aura (likely at the cost of a feat). There is a Paladin MC feat that gives you a defender aura, but there isn't a power swap feat for fighters that lets them change out. However, a defender aura would help you make much better use of powers like this as it makes your mark enforcement an opportunity action rather than an immediate. Talk to your DM about this one.