Monsters don't have to be as balanced as powers. If a monster is broken, the brokenness lasts a session. If a power is broken, it can last the campaign.
And while there are thousands of powers, when you consider that you only get the ones related to your class, you only get the ones in a certain level range, and in many cases some powers are so good or so bad that it limits the pool even more. So new powers is always welcome to me.
Well, I suppose its true, a badly designed monster may be less of a problem than a poorly thought out power.
My point about thousands of powers is that there is really very little reason why a power from one class can't simply be ported to another class. In a minority of cases it may require a bit of retooling due to some specific mechanics tied to a class feature or such, but the vast majority of powers don't have that problem. Obviously a good percentage of powers aren't thematically appropriate for a given target class, but that still leaves you with a LOT of choices. Probably enough that in the majority of cases it can be used as-is. In other cases it may require a slight amount of tweaking, changing of keywords or maybe even a small power adjustment.
I figure it this way. Every class that has a power book covering it has roughly 8 powers of every level. Supposing for the sake of argument you needed a new Wizard Encounter Attack Power Level 1. Just looking at the most likely sources you have 8 warlock, 8 sorcerer, 8 bard, 8 cleric, and 8 invoker powers that might potentially fit mechanically. That's 40 to choose from and there is no real reason to suppose that the other dozen odd classes have nothing to contribute either. I think in the vast majority of cases you'll find something VERY close to what you want. A switch of power source (possibly), class, maybe drop or alter a feature based mechanic, maybe change a keyword or two, write some new fluff, and you're in business.
Honestly at this point it would be pretty difficult to propose a new power and it NOT to have mechanics nearly or entirely identical to something out there already.
The main exceptions I would expect would be something like a summons where the details of what is summoned probably needs to be figured out, but that should be a pretty straightforward swap in terms of the spell itself and even here the chances are you can do a very simple reskin of an existing summons.