From Orberg’s Roma Aeterna, in the second to last paragraph at the bottom of pg. 15
Would this be a good figurative translation of that phrase:
Which will soon be mentioned
The companion to Roma Aeterna has the literal translation as:
About which there will soon have to be a speaking
I don’t know what you mean by “figurative” but your translation misses the force of the gerundive construction. You have turned something which “will have to be done” into something that “will be done” , ie a simple future. The idea is that the Arch of Titus is so important that the author mentions it but then says he will have to talk about it later on in its proper place in the narrative.
I think you already know my opinion about translating Ørberg. You would be better off trying to frame questions in terms of the grammar. You will eventually find that once you have understood the grammar there is no need to translate - unless someone is paying you to write for a Latin-less audience. 
Deleted