What is more grammatically correct? and what are their differences?
What if I come back for you?
What if I came back for you?
Same goes here:
What if I told you...?
What if I tell you...?
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They're all correct, but they have different meanings.
"What if I come back for you" is an offer. "I am offering to come back for you, if ...".
"What if I came back for you" is weaker, a request for information. "Tell me what you would do if I came back for you, then I will decide". It's hypothetical.
I think in many cases the difference in meaning is subtle and not very important.