Making your martial controller an "Exotic weapon Specialist" might let the player have a reasonable amount of control (pun incidental) of how they balance the controller elements...
Look at the multiclass options for whips and chains for ideas of powers.
Maybe each weapon comes with a set of powers, so when you choose a weapon, you are also choosing your at will power. At will powers also let you draw the relevant weapon as part of the attack action Then encounter and daily powers need you to be wielding the right weapon

Net - control of multiple targets in an area at range. At Will - "Cast" - range 10, minor damage, immobilise and optional pull.
Whip - short range control with damage. At will "A lick of the cat" Short ranged, single target, bonus to hit
Chain - Damage multiple foes at fairly short ranges and melee. ""Chain punch" Melee 2, damage
Grenades - pretty well discussed above. The at will you choose gives you grenade proficiency and the ability to make and use the appropriate grenade. Eg "Frost grenade" gives proficiency in grenades and the ability to make frost grenades". Encounter and daily powers allow more exciting greandes