Okay, odd request.

I love Creatures 1 and 2. Love the artificial life aspect, love all the stuff you can mess with, and love the modding opportunities.

However, I also really love the atmosphere.

Thing is though, sometimes I wanna play a more traditional game, and not... well, a virtual biology lab.

So are there any RPGs/adventure games/action games that have a similar atmosphere/world-feel to C1 and C2?

I can't think of a game that would fit. Maybe a kind of open-world side-scroller RPG, preferably with nice, seamless graphics? Basically, a Metroidvania in a natural setting. There's Terraria (and its in-development, moddable, spiritual successor Crea), but I wouldn't say it has the same atmosphere as Creatures. Maybe if you specify what you like exactly about the atmosphere, it will be easier to find something.

(And if it doesn't exist, let's make it XD)

Oh, there's also A Valley Without Wind, which fits at times. I say "at times" because the art direction is a bit random--it can look very good in one place and really ugly in another. Which is a shame since it's apparently a great game. There's a sequel being made, and it looks much better.