のではないでしょう / だろうか and じゃないかな

Adding one of the following phrases at the end of a sentence can express the speaker's/writer's conjecture or opinion on a matter in an indecisive fashion:

• のではないでしょうか
• のではないだろうか
• んじゃないかな

The first two are only used in written language, with the second being the more formal of them. The third is obviously more casual. It often translates as something like "I think that..." or "Isn't it that...?"

地球温暖化問題はもっと大きくなっていくのではないでしょうか。
Global warming will continue to become a bigger problem, will it not?

もっと日本語を勉強したほうがいいんじゃないかな。
I guess maybe I should do more Japanese studying.

あのクラスは、毎日宿題や小テストがあるので、大変なのではないだろうか。
I think that this class is tough, since we have homework and quizzes every day.

猫と住んでるはいいんじゃないかな。
Living with a cat is nice, I think.