Why is there no future perfect subjunctive verb form in classical Latin? I can't think of a time it would be used, but I can think of an English translation: "if subject were to have verbed, then clause". I've tried looking it up and I can't seem to find it, so I assume it is non-existent. Why is this?
Latin does have something that resembles future and future perfect conjunctive (subjunctive): the periphrastic conjugation in conjunctive. The periphrastic present forms are formed from the present tense forms of esse and the future participle (with the gerundive playing the role of a passive future participle). For the verb canere, in indicative we have canturus/canendus sum/es/est and in conjunctive canturus/canendus sim/sis/sit (and similarly in plural).
The periphrastic present indicative is semantically close to the future indicative, but not quite the same. In active voice periphrastic forms imply that someone is about to or is planning to do something and in passive it implies that something should be done.
The closest thing to future conjunctive is the present tense conjunctive of the periphrastic conjugation. The future perfect conjunctive is realized via the imperfect conjunctive. These are used in subordinate clauses as required by consecutio temporum:
- Scio, quid canas nunc. I know what you sing now.
- Scio, quid cecineris heri. I know what you sang yesterday.
- Scio, quid canturus sis cras. I know what you will sing tomorrow.
- Sciebam, quid caneres. I knew what you sang (while I was there).
- Sciebam, quid cecinisses. I knew what you had sang.
- Sciebam, quid canturus esses. I knew what you were planning to sing.
The passive periphrastic form cannot be used this way, though. In a passive subordinate clause future and current events (from the point of view of the main clause) are both indicated as current.
- Scio, quid canatur nunc. I know what is being sung now.
- Scio, quid cantum sit heri. I know what was sung yesterday.
- Scio, quid canatur cras. I know what will be sung tomorrow.
- Sciebam, quid caneretur. I knew what was being sung.
- Sciebam, quid cantum esset. I knew what had been sung.
- Sciebam, quid caneretur. I knew what was going to be sung. (Future should be indicated by adverbs to make it clear.)
It's not just Latin. As far as I'm aware, the only language that has a future subjunctive is Spanish, and it's disappearing there as well. (I don't speak Spanish, so I can't say from personal experience.)
William Harris, in Orbis Latinus, writes that the subjunctive calls forth
all the associations that go with unreality, possibility, potentiality, in the English words "may" and "might" and "could be" and " if it were...". These are in a different world from the world of fact, where things "are", where "is" can be counted upon to "be", where facts are facts when you get down to brass tacks.
The subjunctive mood deals with things that may not actually happen or have happened; since there's no way of knowing whether something will or won't happen in the future, the idea of subjunctivity—of possibility rather than certainty—is already inherent in the future tense.
As TKR said, there's also no future active subjunctive. Although I don't know of any particular historical reasons for it (probably just another language quirk), I suppose the future more vivid construction somewhat accomplishes what a future subjunctive might.
For example, with the phrase Sī Marcus Iūliam amāverit, ea eum amābit, translating to If Marcus loves Julia, she will love him, the future more vivid uses a future perfect verb in the protasis and a future verb in the apodosis to express something to the effect of "If Marcus should love Julia (sometime in the future), she would love him," which has somewhat of a subjunctive-like feel to it.
Alongside Joel's and Nick's answers, I'd like to call into question your English presumptions. To my ears, there's no different between "were to have" and "had", i.e. the pluperfect. Consider the following examples:
If Marcus were to have been king, he would have killed them.
If Marcus had been king, he would have killed them.
I can't see a difference.
The future perfect needs actions in the future before other actions in the future. I don't think "were to have been" accomplishes that at all, because were is past tense completely. Your sentence is thus a past contrary to fact condition. For completion's sake, for a past contrary to fact condition, you use the pluperfect in both the apodosis and protasis.
Si Marcus regnavisset, illos interfecisset.
Since your question concerned a conditional clause, I'll focus my answer there. In a conditional clause, the present subjunctive refers to an indefinite time, including the future. Si dicas does not (usually) mean "If you are now saying," but rather, "If you should (at some point) say."
If the condition refers to a specific future time, the future or future perfect indicative is used. Thus, there's just no need or room for a separate future subjunctive.