hmm well how about:

It can take place in the original universe. Plot would be ..well i have some ideas.

1. Mission gone bad. Basically Yang or Ruby is fatally injured where the other has to care for the uninjured and the experience draws them closer as sisters and Yang or Ruby eventually confess even tho the possibiltiy of rejection is strong.

2.Birthday/special occasion: During the event one of the twos feeling come to light and unable to hold back anymore a confession is due.

3. Love is War: Based on the vocaloid song. Yang and Blake or Ruby and Weiss where Ruby or Yang attempts to break up the romance with Weiss or Blake by any means to get the attention of the other. more drama based.

or if you have any ideas Vile

I also have another idea that Yang and Ruby are forbidden lovers hiding thier relationship but when it is found out..will thier love be strong enough to prevail?