To my mind, Imagine if you died omits the Direct Object; it reads:

Imagine [what would happen] if you died

which is slightly different than actually imagining your death (Imagine you died).

The version with if technically focuses less on the actual event and more on its consequences.

But in spoken language, the pragmatical meaning is probably the same. Just imagine this situation: what would happen? What would you do? How unfortunate would that be etc.

PS. While researching, I found rare examples with if in the dictionaries. So yes, definitely a colloquialism.

Here is one from Cambridge:

Imagine if you were suspected of a crime and police wanted to know where you were at 5 p.m.

To my mind, Imagine if you died omits the Direct Object; it reads:

Imagine [what would happen] if you died

which is slightly different than actually imagining your death (Imagine you died).

The version with if technically focuses less on the actual event and more on its consequences.

But in spoken language, the pragmatical meaning is probably the same. Just imagine this situation: what would happen? What would you do? How unfortunate would that be etc.

PS. While researching, I found rare examples with if in the dictionaries. So yes, definitely a colloquialism.

Here is one from Cambridge:

Imagine if you were suspected of a crime and police wanted to know where you were at 5 p.m.

Also, in addition to the Ngram @Kate provided, here is a comparison between the two variants. You will find that imagine that is far more common still.