I sometimes build monsters off of the PC's backstories. I'll use 13th Age as an example, because backstory elements are a major part of the game and every PC has them on their character sheet. But any game where you've got player character backstories will work.
For 13th Age PCs, I look at their one unique things, backgrounds, icon relationships, and whatever weirdness the players have latched onto, and try to build monsters out of this strange stew. I don't use every backstory bit every time. I pick 2 or 3 for my monster mash-ups.
First I figure out what level, and then I mash up two monsters of that level that have some tie to my PCs backstories. There's a great master monster list you can download here. Or just use the chart in the back of the Bestiary.
For example: Do the PCs have lots of icon relationships with the Orc Lord? And a PC whose background is an ooze and slime collector? I look for monsters associated with the Orc Lord and ooze or slime at my target level. I think it must be time for a 4th level Gelatinous Orc! Simply take the best features from both, and use them.
If I take a 3rd feature from a PC, I'll take one that is just for flavor, like Cat Whisperer of the Elf Queen's court. A little gelatinous cat sits on the monster's shoulder, but has no mechanical effect.
Hmm, I can't find any 4th level orcs, but a bugbear will work! A gelatinous bugbear! I take stats from the Barbarous Bugbear from the Bestiary. (I could level an orc, but if I'm improving in game, I don't have time.)
And the gelatinous tetrahedron, also from the Bestiary. I better bookmark both pages.
I take the higher value of two out of the three defenses and for HP and initiative. I take the lower of the two for one of the defenses. I chose the lowest of the defenses from either monster and use that. I like to keep the rock, paper, scissors (armor class, physical defense, mental defense) design of 13th Age combat going.
I'll write down the init, defenses, and hit points. So, AC: 18 or 19 -> 19; PD: 17 or 17 -> 17!; MD: 16 or 14 -> 14; HP: 51 or 160 -> 160. Init +12 or +3 -> +12. I'll make the size huge for the gelatinous special effects. I might give small boosts to PCs with relationships to the Orc Lord or with ooze and slime backgrounds, say +1 to hit.
"It's a huge bugbear made out of green gelatinous goo. On it's shoulder sits a gelatinous cat who squints at you."
I'll just flip back and forth in the Bestiary for the attacks.
Bugbear stuff: The ferocious start is cool, I'll use it. 2 attacks a round to start with, yay! I'll also use the club attack from the barbarous bugbear, calling it a gelatinous club, but I won't use the natural even hit or miss power. I'll save the special effect for the gelatinous part of my creature.
Gelatinous stuff: I'll say an even gelatinous club hit triggers the gelatinous tetrahedron's engulf and dissolve, on an odd hit it does the club damage from the bugbear. "The gelatinous club splats all over you and covers you in slime, drawing you inside the huge jelly-like bugbear. The gelatinous cat licks its lips."
I think I'll leave out the gelatinous tetrahedron's instinctive actions. I'll save those for the full-on gelahedrons.
Nastier Specials: For extra scariness I like to ask my players. "What's the worst thing about a gelatinous bugbear?" One player is sure to blurt out something awful. Probably about the gelatinous cat spitting slime balls. The rest will moan and beg them not to say things like that, but really, everyone is enjoying the fun. If no one offers a fun suggestion, I'll choose from one of the base creatures or make my own (slime ball attack on a natural even miss!). Crits on 18+ is easy. 16+ if I'm in the mood.
See Also: The undead owlbear archaeologist.