One type of aid other in there makes the whole process more reliable...
doesnt actually extend the end result as much as some others --> Mechanical effects might allow an aided other (or all in the party in a skill challenge) to roll 3D6 or 2D10 or even taking 10 when you cannot normally
Another might increase the quality of end results in an erratic way ... perhaps adding d6 to an allies rolls
Some might be able to add a small bonus to everyones roll in a skill challenge
Still just throwing out options
How about extending the combat roles into other aspects of play? Leaders get ways to assist more effectively, or increase the effectiveness of other character's assistance to each other. Controllers acquire abilities to manipulate the environment in different ways that aid the party. Defenders get to limit the opposition's options and force them onto the horns of a dilemma, and strikers are able to focus themselves on achieving specific goals (IE maybe they have ways to get better bonuses on checks).
Now, maybe this is too restrictive and imposes too much structure on various archetypes, but I think its worth considering (IE are their ways that paladins, fighters, wardens, and swordmages can all "limit the opposition's options" in ways that are thematic and span across the range of different situations that are likely to come up). This might be an area where 5e "pillar theory" might prove useful, though I'm not a big huge fan of it in general.
I'm imagining something like rogues have their SA dice in combat, they have sneakiness and deception focus that can add to most of their checks in exploration mode, and they have deception and street smarts during social situations. All of these do basically the same kind of thing, add to the 'advance the success of the encounter' measure, whatever that happens to be (in combat its extra damage, in a social situation SC it might be generating advantages). You might rework the SC system a bit so that advantages aren't just handed out automatically anymore, instead strikers get features that generate them explicitly. For the rogue this might be something like they can declare a specific social skill check to be one increment easier if they pass a medium check (with some narrative justification of course, but the DM should look to provide room for this sort of thing, much as he might make interesting terrain features available in a fight).
Fighters might be able to do something similar in situations where some kind of physical prowess is required in an exploration type SC, or exercise some sort of guarding function in a social one, so that maybe they can get a check of some kind to thwart some countermove by an enemy that equates to, again, an advantage being granted such as a reduction in a check's difficulty, etc. For a swordmage this would be flavored as a use of defensive magic, even a byproduct of the Aegis they have. For a paladin it would be some sort of divine assistance (maybe a poison simply fails to work on your ally, or a hidden threat is revealed). I think you can do the same with other classes.