My dream come/s true

It's somewhat of a frozen formula idiom, T and as such you'll most often hear dream come true.

My dream comes true is not a likely occurrence because we don't normally use the present simple to describe one time events. The present simple describes more the routine/habitual/ general/scheduled condition.

It's not impossible though. It could be used to describe an event in a play, which in essence, describes an event that has become fixed.

Or it could be used to relate a past story/narrative where we use the present simple to add currentness/importance to the story.

should is rarely an apt description of what makes language work. We use differing structural forms to effect different meanings and to cover differing life conditions.

A longed-for event which eventually takes place is often described as a "dream come true", which is a compound phrase having the effect of a noun. You have to imagine the words "which has" or "which had" or "which will have"(as appropriate) between the words 'dream' and 'come'. Other phrases like this are a dream gone bad, a plan gone wrong, a johnny-come-lately (an upstart, a person recently arrived who acts as if he has been here a long time, to the annoyance of those who have).

However, you could say "My dream comes true next Tuesday when my new Ferrari is being delivered."

Many thanks, JTT and Contrex, for your helpful guidance.