When "I, Tyrant" was released and I was going through the beholder stage of my life, I planned out the idea for a campaign based around a new beholder city that had located near established settlements - similar to the accompanying modules for that series, but on a much larger scale.
The campaign was going to start with a beholder invasion of the players' city. They start hunkered down in trenches as they first spheres appear out of the early-morning mist... Yup, 1st level characters against beholders and beholder-kin (albeit with a few thousand NPC soldiers to add to the excitement). Think of eye-rays scything out of the mist, panic and screeming all around - I had WWI images in my mind whilst thinking of it all.
So the players get some prepared defences (nets, archers, pits to hide in to surprise the beholders from underneath), but after that they're on their own. There's a few problems with this approach (mainly the fact that you have to be somewhat protectionist of the players, but then again that's what the meat-shields - urgh, sorry, NPC soldiers - are there for), but the images this idea brings into my head still make me think this is a great idea. Street-to-street fighting with beholders - yum!
Never thought it all the way through though and consequently never played it, so it's probably chock full of loop-holes.