Verb stem + verb? is に omitted or has the Vstem independent use?

来る isn’t the greatest example since it’s an exception to many things.

Only Godan Verbs 五段動詞 (Group 1) have an imperative form that uses something that would look like a stem, i.e. the -え form.

e.g. 行く→行け。飲む→飲め。返す→返せ。