Come to me, and I'll give you a fight you'll never forget.
The subject of the example sentence above is you because a 2nd person command forms the conditional clause (subjects get dropped with commands). It's perfectly fine to use the imperative to form the condition, and identifying what the speaker might be implying does not change that (i.e., that he is an "interesting fighter").
You could also use the present indicative for the first clause:
If you come to me, I'll give you a good fight.
If is also a conjunction, so the conjunction does not have to follow the condition, and there are a variety of other verb tenses and moods that can be used in either clause. It is not one rule, but a system of rules in that determines the verb tense and mood. Following an imperative statement with the future tense is just one way of forming a conditional
You could also use the subjunctive followed by the conditional, though sometimes you wind up with a more poetic or slightly archaic sound:
If I were a fighter, I'd fight you.
If you were to come to me, I'd give you a fight that you'd never forget.
The subjunctive is falling out of favor in English though, so you might try the simple past tense for the condition:
If I fought you, I would put you in the hospital.
You can find many other examples off-site. You could try the Wikipedia article on conditional sentences in English or scan through university pages for examples, being careful to verify the source if you are concerned about having authoritative information: http://grammar.ccc.commnet.edu/grammar/conditional2.htm