You seems to be pretty experienced in Chinese. Great! But your impression on imperative form of Chinese is not entirely correct.

1. Most sentences with "we" and "you" as the subject can be used as an imperative sentence. But there are some intricate intonation difference between a declarative sentence and an imperative sentence, which I can't describe in a few words (but you will get it after practicing Chinese with native speakers). It's not recommended for Chinese beginners.
2. Contrary to what you may believe, an imperative sentence can omit the subject if the subject is obvious. And people do it in most cases. Omission of subject is the most obvious feature of an imperative sentence.
3. 吧 indicates the hortative mood. which softens the feeling of "commanding" a lot. But it is optional.
4. Beside the omission of subject, another difference between the imperative and declarative sentences is that you need to use 别 / 不要 instead of 不 for negative imperative sentence.

Yes, very much like “Look!”, “看！” is also valid.

I think that's fine for the imperative form. You certainly wouldn't be wrong to use 吧. Personally, I would use 看一下，看看，or看一看 at the begining of the sentence if I wanted someone to "take a look" at something. However, those versions of 看 suggest a certain amount of leasure, and the particle 吧 is a suggestion, not a hard command, so they would not be suitable for an urgent situation like "Look! It's on fire!"