Tacitus Arctous dixit:
I am not sure it has to be the future, unless the main clause shows meaning of future.
For example:
"ego in Arcadio opperior, dum ista cognosco"
"Tityre, dum redeo, pasce capellas"
"retine, dum ego huc servos evoco"
True enough, but in each of your examples the direct, literal meaning of
dum may still be construed as "while", although the end of the duration is clearly the point of emphasis. (Indeed, from these one can easily see how
dum came to mean both "while" and "until".) Yet this is only possible in these sentences because the verbs within the
dum clauses are all imperfective in aspect, that is to say continuative or durative, and thus each is conceived as occurring concomitantly with its main verb: when the one is fulfilled and ceases, so too the other must cease.
To illustrate, in the first sentence
cognosco has the imperfective meaning "I'm inquiring about" or "seeking to learn" rather than perfective "I know" or "have learned", which would on the contrary have to be expressed with the perfect tense. Hence altogether it literally means: "I'm waiting in Arcanum [not "Arcadum"] while [i.e. only so long as] I inquire about these matters (of yours)". Implication: once he's found all the answers he was looking for he'll leave.
So also with the second sentence: "Tityrus, while I'm returning [i.e. have not yet returned home], feed the she-goats." The act of returning is conceived as a process rather than a single action.
And the third: "Hold him while I call the slaves hither." Here the English is expressed no differently than the Latin. The clear implication is that it's not until the slaves
arrive that he may let go.
Is this clause a future in meaning? To me "exsurge" here is just a present command to rise up.
Correct me if I am wrong.
The future idea is contained in the "until" clause, not the imperative. Though I'm not sure I've quite grasped the full import of the request, I tend to agree with Bitmaps that there seems to be an idea of purpose in it, in which case the present subjunctive is called for. So maybe something like:
Exsurge identidem usque eo quoad oves in leones abeant.