Student Council
During his first year Hikigaya decides to apply for a position in the student council after Shiromeguri Meguri is elected as he thinks it could help his future cv and be good experience.
Getting close with Meguri here would be nice and she can become a big sister figure or possibly something romantic.
You can decide whether this would lead to Meguri wanting Hikigaya to run for president next term after she steps down or not.
You could combine this idea with the 'Different Trio' one and have Hikigaya be friends with Totsuka and Kawasaki in his class and avoid any Service Club drama in the future.
