As a writing director for Moonshae Isles in the Living Forgotten Realms campaign, I do see a number of adventure proposal hit my inbox. In 4e, powerful monsters like beholders, demons, devils, dragons, drow, and mind flayers. Some of the staples of earlier editions high-level adventuring are now reduced to a series of thugs that can be met at any levels. In spite of that, they can (and should) still be used as in more classic roles.
I personally like seeing creatures "that makes sense" and follow themes when building up encounters and strongly encourage my authors to do the same. If a monster appears, it must have a good reason to be there with its buddies. If playtesters or I don't go "why is that guy there?" the monsters make sense.
Some time that raised eyebrow is exactly what the author is trying to accomplish however. "Why is that treant doing deep in the Underdark?" As long as the adventure explains it, its fine.
In LFR, the necessity of sticking to monsters published in WotC supplements does limit what you can put in there. But to be honest, with Monster Manual 2 (IMO a superior product to the lackluster and grossly incomplete MM1) the variety of monsters has greatly improved to a point where this is much less of an issue. My issue is in my wallet however...
When thinking of adventures and encounters, using bigger and usually cooler monster is a definite draw. However putting something interesting around them is often a different story. Taking a big monster and building an adventure around it is usually where the process fails and adventures just become well... a long and boring series of encounters leading the PCs into the jaws of the monster. Turning a good monster into yet another thug: flavorless and far from memorable. And 4e has more than its share of thugs already.
So... how can the "big monster of the week" or "shiny new big monster" be used to create something good? With the high-volume release schedule that Wizards is pushing with 4e, the urge is strong to publish more and more things and have a constant "weekly guest star" in your adventures. So how can you merge the two?
- Use the setting as much as possible. Instead of setting the monster in a "previously unknown region", put it where it would affect some pre-existing location in the campaign. This immediately places the beast somewhere. A dragon arrives to terrorize the village some time after the PCs have left.
- Retcon the monsters' ties to the setting. Retcon= Retroactive Continuity. Is a process by which facts in a work of fiction are deliberately changed to suit the current story better. Examples are many in comic books, but the most famous was in Dallas where they erased one full season to return Bobby by having his wife wake up and having dreamt the previous season... Here you have a group of bad guys already present. Well the monster is the puppet master behind that group. Try to avoid doing this too often or your campaign will turn into one of the bad movies by M Night Shyamalan (however his name is spelled, the guy from the Sixth Sense) its fun the first time, but after one... It gets old quick.
- The monster's resources should be realistic. Minions and servants don't just materialize out of thin air. Orcs and goblins horde take years to reform. If the PCs have just destroyed the main orc horde, there new warlord should not have the same amount of troops two weeks later. He might want to spend a few years consolidating his power before coming back.
- Use foreshadowing. Do not immediately use the monster, allow it to simmer and make its place into your campaign. Have the PCs find out more about it and finally discover what it is. They will love you for it! Okay, I know this is like telling a 4 year old "do not eat the candy" while leaving the room for 3 hours. But if you can resist the urge, it will pay off.
- People know. If there is a monster out in the wilds... people in the area should know about it. One of the common mistakes I've seen is that a huge dragon flies in from nowhere, builds a huge base with a horde of minions just 2 miles out of town and no one knows anything about it.
People know. And if they want the PCs to help, they should be willing to tell them about what they know. Of course, exaggerated tales or partially erroneous tales are fine, but people should know about a large menace nearby.
In the end I guess that big monsters should be used in a way that enhance the campaign. Their appearance mean something in the grand scheme of things. Using them as turning in your campaign will give you the most satisfaction out of your monsters.