Hey there,
I just finished Sassy Go Go and am currently working my way through Reply 1997 but I feel like I need more dramas to watch alongside, that are easy to finish or marathon because of how short they are and how easy they are to watch :)

Is there any other dramas no longer than 12 episodes that is really addictive with an amazing storyline and great casting? :3
Try Because it is the first time! Its also a mini series like 8 Eps!
Oh sounds good! I didn't know it was a short drama, that'll definitely be good to watch :)
Splash Splash LOVE is only 2 episodes and is worth every second of the 120 minutes it takes to watch it. 

Noble, My Love is really cutesy and fun. It's 20 episodes, but each episode is only 15 minutes. So, it's more like 5 regular episodes. :)
Oh I forgot about these! I will definitely have to put the time aside to watch them, they sound really cute! Thank you for reminding me haha!!
The Producers (already in your PTW) is a great 12 episode drama :)
Aftermath
Himitsu no Kankei - Sensei wa Doukyonin
Juunen Saki mo Kimi ni Koishite
Ma Boy
I wasn't sure about The Producers originally as I heard mixed reviews on it but I think I'll pick it up anyway at some point :) 

Thank you for the recommendations! There are probably alot of Japanese drama that are very short too :3
Sirius
Twenty Years Old
Surplus Princess
White Christmas
Dr. Frost
Liar Game (Korean, since this is for Korean dramas, but the Japanese one is good too!)
Vampire Prosecutor 1 & 2
Monstar
To Be Continued
Vampire Flower
Girl of 0AM
Looking Forward to Romance
Missing Noir M

I don't know about "fantastic cast" and "addictive story lines" for all of them, it depends on your taste, but they're less than or are 12 episodes and I enjoyed them despite their small amount of episodes. :')

I would recommend Love Cells too, but it's slightly longer than 12 episodes restriction. The episodes are only around 10 mins each, and I think it was less than that. 
I recently watched Missing Noir M and all I have to say is WOW
Also Splash Splash Love is the best special in existence, so simple!!!!! <3 <3
does it have to be korean? proposal daisakusen is japanese drama 11 ep and i absolutley loved it
High-End Crush has 20 episodes but each episode is 15 minutes so I think it fits into what you are looking for.  It's a light cute rom com that I really enjoyed.  Also, I know a few people have recommended Splash, Splash Love and I totally second that.  It is one of my favorite dramas--its cute and funny and just all around awesome,  Definitely give it a try.
Adolescence Medley
Bad Guys
Orange Days
Yeah, forgot to say anything with short episodes also are welcomed (: EXO Next Door for example was fifteen mins each roughly adding up to 16 eps but easy to watch in short chunks!

High End Crush is definitely on my watch-list, as is Bad Guys.
White Christmas is AMAZING and I still need to watch the last episode haha (I am terrible)
Hadn't heard of Missing Noir M and I should really watch Splash Splash Love soon as it's a really quick watch! (I keep forgetting haha)

Thanks for all the recommendations :D I really appreciate them and will add some to my list that I hadn't heard of too!
Special Affairs Team TEN and you should also look to japanese dramas.