A nice idea and i has been suggested in the past but as you mention its difficult (if not impossible) to implement with the current game engine
as to your OT question no its not unheard of and it has happened most recently with EVE but its no small task and can cause a lot of problems (as it did with EVE) however i wouldn't be surprised if the NCNC intend to do this at some point in CoH future.
This is basically already in game.
Controllers can do extra damage to held targets. It doesn't matter who inflicted the hold.
I think the main issue would be that every potential power overlap would need to be configured individually, which would take a fair amount of resources from the Powers team.
Disclaimer: The above may be humerous, or at least may be an attempt at humour. Try reading it that way.
Posts are OOC unless noted to be IC, or in an IC thread.
Oil Slick was an attempt to introduce this sort of thing. I think the problems they have had with it has put them off doing more of the same!
I really should do something about this signature.
In some ways we alredy have this. In the secondary effects. Like -ToHit, -Resist, and -Defence.
The /sonic defender would soften the armor of an enemy for the /SS tank. Or the Dark/ corruptor would distract the enemy so it can not hit the /SR brute.
However if Ice powers makes electric attacks more damaging, then I think it will properly just lead to more Ice/electric dominators (and the like).
DC Universe Online is going to be doing this, so if DCUO makes it to launch and this proves a popular feature then it might find its way into CoH/V.
Also don't forget that stalkers used to be able to combo with others too back before the stalker upgrades. Before they're attacks had a chance of crit if they're target was slept or held so dominators and stalkers used to make a good combo, in fact they still do today 
As PRAF68 said, Oil Slick was their attempt at exactly what you're suggesting. And... well, Castle now refers to it as "that power" after twitching nervously. I wouldn't count on more of the same. Unless one of the new guys at NCNC volunteer for THAT suicide mission, that is.
Necrobond - 50 BS/Inv Scrapper made in I1
Rickar - 50 Bots/FF Mastermind
Anti-Muon - 42 Warshade
Ivory Sicarius - 45 Crab Spider
Aber ja, nat�rlich Hans nass ist, er steht unter einem Wasserfall.
I have been thinking again.......uh oh i hear some groan. But with the combination effects available within the dual blades set I thought it would be really cool if some different powersets on different toons could interact with each other.
For example a foe is immobbed in a stone cage or something similar and the super strength tank then uses knock out blow and extra damage is awarded. Or an ice blaster/def/troller uses ice patch and then the electric blaster fries them and extra damage or end drain is awarded or a fire based toon hits them and extra damage is awarded.
I believe there is plenty of possibilies for interaction between powersets. I am sure this would be difficult to implement but comic book culture is full of examples when different heroes combine there super powers to gain an extra effect or boost the damage etc (fantastic four etc.). Like I said it would be a cool feature and if impossible to implement so be it, but would deffinatly be on my wish list for CoX2.
OT Question: Is it unheard of an existing game getting a makeover of the engine so that features like the above could be included or is stuff like that saved exclusivly for sequels?