Learn Dutch: Kom alsjeblieft hierheen.

Hier just means "here" as a fixed location, as in "I am here." Because the verb kom involves movement towards a place, Dutch requires a directional word. Adding -heen (towards) to hier creates hierheen, meaning "to here" or "this way."

Could I say hiernaartoe instead of hierheen in this sentence?

Yes, you absolutely could. Both hierheen and hiernaartoe mean "to here" or "towards this place." They are fully interchangeable, though hierheen is a bit shorter and very common in everyday speech.

Why is there no subject (like jij or u) next to the verb?

This sentence uses the imperative (command) form. Just like in English, when you directly tell or ask someone to do something, you drop the subject pronoun. The imperative form in Dutch is simply the stem of the verb, which is kom.

Why does hierheen go at the very end of the sentence?

Directional adverbs like hierheen and daarheen almost always go at the end of a simple main clause. They behave similarly to separable verb prefixes, getting pushed to the very back of the sentence.