There are a few typical models for expansions:
1. Grandiose addition. Something like In The Lab addition to Pandemic or Cities and Knights for Catan. A new board is added, lots of new components, new actions can be taken, a new route to victory is opened. Basically, what this expansion adds is not able to be ignored -- it changes something fundamentally about how the game is played. I'm not a big fan of these sort of things as I usually like the base game just fine and these additions usually up the complexity without an equivalent return in enjoyment.
2. Variety addition. These expansions change absolutely nothing about the base game, they just add more variety. Hidden Signs for Mysterium or Dixit expansions are examples of these. I enjoy things like this. It can get old seeing the same old cards after the 20th time you've played a game -- these help breathe a bit more life into something that may have gotten stale for you.
3. Variant addition. That's the Best Part for But Wait, There's More! is an example of this -- a tiny addition to the game that you can add. It doesn't fundamentally change how the game is played or add incredible amounts of complexity, and most of the time you can start out new players by teaching this to them and they'd never realize it wasn't supposed to be there in the first place. Sometimes these can really spice up a game or help overcome a small weakness or dominant strategy the original game had. Sometimes these are at a really low price point or exist as promos.
4. Combo addition. This is a mix of the above. On the Brink for Pandemic or Traders and Barbarians for Catan or Old Saloon for Bang The Dice Game are examples of this. You grab a few variants, some extra cards, and maybe 1 big addition and throw it all in a box. Usually the collection of variants are modular so you can take or leave them. Seems like you end up getting a bad variant here or there that few people like (Bioterrorist) but most of the times these sort of expansions are well received because they really flesh a game out and allow you to get a lot more options out of a base game with a single purchase.
I didn't expect much from the Cthulhu monster for King of Tokyo. I like Cthulhu. I like King of Tokyo. Seems like kismet. What I got was something I didn't realize: a really great addition for $10.
On the most basic level you get a monster standee (now I have 14 standee bases... for 14 player KoT?) and his card. Out of the box that would allow you to play King of Tokyo or King of New York with a new monster. Not much different from various promos the game has had before. In addition to that, though, you get a few other things.
There are Evolution cards (aka Power Up cards) that you'd find for the other characters in either King of Tokyo or King of New York's Power Up expansions. So if you are playing with the Evolution cards (which you should) in either game you can have another option. He also comes with madness tokens that come into play with some of his Evolution cards as well.
In addition to that he comes with something I am sure has been done in other games, but I don't know any off the top of my head. They are cultist tokens which are essentially a variant for King of Tokyo. They add a little bonus that you can get occasionally that you can throw into a game or leave them out.
The reverse of these tiles add buildings to destroy in King of New York (which then turn into cultists) -- something you can mix into your existing buildings or leave out if you wish.
It is a bit difficult to explain why I enjoy this so much and I feel like it is such a great idea for an expansion. The closest similarity for me would be a role you could get that was able to be used in Pandemic and Pandemic Iberia and sets of tokens that were used as a different variant in each game.
Having a $10 expansion come with 2 different varients for 2 games and a new character that can be used in 2 different games is great. Obviously the stock goes way down if you only have 1 of the 2 games, but if you DO -- it is a great idea.
I'm a fan of small variant expansions to begin with, and if there are more like this with small price points that are flexible and reuse their components in creative ways -- I'm there.