Q:

Tense usage? What to use in future situation?

I'm confused by the conditional tense in French, for instance...
"Dans l’avenir, je ne serais pas surpris si les nations disparaissent et sont remplacées par les organisations a l’échelle planétaire."
What would be the correct tenses of the bolded verbs? Would they all be in the conditional tense, or would they be subjunctive (since its hypothetical?) Or...

A:

I am not sure why you talk about the subjunctive since there is no subjunctive in the sentence you give.
It is, as you correctly tagged and used, a conditional.
With the conditional, you have to use the indicative present, as you did, or the imparfait tense, which are both correct sequences of tenses.
Here it is with the imparfait, which appears more natural to me, although your example is also correct:

Dans l’avenir, je ne serais pas surpris si les nations disparaissaient et étaient remplacées par les organisations a l’échelle planétaire.

A:

This is just an extension of Subtenante's answer:
And if you replace si by que (I wouldn't be surprised if becomes I wouldn't be surprised that), you have to use the subjunctive:

A l’avenir, je ne serais pas surpris que les nations disparaissent et soient remplacées par les organisations a l’échelle planétaire.

