That's a mistake. If there's anything people need guidelines for, it's how to improv. Without structure most people are paralyzed by choice gloat, and since 4th edition has a very good structure, people feel paralyzed without it. This is a truly amazing aspect of 4th edition that is so underutilized that it's a bit unfair to have even an opinion on 4th edition without trying it.
So here's the guidelines that my group and I have started to throw together. They're not perfect, but they'll work. All final decisions are to be made by the DM, appeals should be few and far between.
1. Determine the type of action it is. Is it standard, move, minor, free, or any of the interrupts? The action can combine up to two of these types. Any more and it starts getting ridiculous.
2. Determine if the action should use up an Encounter, Daily, or Utility (Encounter or Daily) power. Sometimes your players want to use an ability that's truly over the top. Instead of trying to tone it down, ask them if they're willing to give up an encounter or a daily power to use their idea. Discuss this with the players, and most important of all listen to them! All it takes is screwing this up once for people to get the idea that you're inflexible, and they won't want to try again. If the player is out of encounter or daily powers then do not allow them to use their action. This is where things stay balanced, because a character is only able to be that awesome so often.
This is where the structure of 4th edition is extremely helpful, because as the characters get more powerful they get more powers, and thus have more slots to mess around with. The players will actually feel their power increasing because of their ability to make up more stuff more often, with their established powers as fall backs in case they can't think of anything.
3. Work out the details. What defense will the power target, if it's an attack power? What should the exact effects be? DM's, I suggest you base whatever scale you're using be off of the following:
- No more than a +2 bonus or a -2 penalty to attack rolls, regardless of who the beneficiary/target is.
- If they're looking for a damage boost do not grant anymore than their second most powerful modifier at heroic, secondary+3 at paragon, and secondary+6 at epic.
- Do not allow for stun as an at-will, ever. Just don't.People will abuse the hell out of it, and the fight will drag on and on.
- These are the most troublesome to make up, because they're right in the middle. When it comes to assigning increased damage, look at the character and his encounters. Consider his class. If you have a Warden, he probably shouldn't be getting too many 2[W]+highest ability powers, should he? If that's something they're trying for ("I don't want any effects, just give me damage!") that's a bit different. Give them a 3[W]+highest ability that targets AC, and move on.
- I would still hesitate to give anyone a stun move at this point. If they insist, reduce the damage to 1[W] and only have it last til the end of their next turn, and even then it's only a single target. Never give a multi-target stun, no matter what tier you're playing at.
- This is, by far, the easiest thing to make up stuff for. Go for a minimum of 2[W]+highest modifier damage for any Daily. Anything lower and the effect had better damn well be AWESOME.
These are rough guidlines, and, with the exception of the multi-target stun admonishment, treat them as such. Making up powers is more of an art than a science, and more haggling than any of the two. Be prepared to make mistakes at first, and make sure to inform your players of that. They'll understand and, if not, they probably shouldn't be in your game anyway. Give this a spin, experiment, and give me some comments on what you find! I'm very interested in feedback.