Is this a fictional soldier? What rank? Is he a simple miles, or a specialist immunis, or centurio, or tribunus militum? Will the inscription be in Latin? Is it to be a funerary epitaph?

In general, for miles citizens in the Imperial period, we see a list of their century and name of their centurion or other commander, not a mention of their cohort. So for instance, on a silver bowl from Vienne:

C(ai) Didi Secundi mil(itis) leg(ionis) II Aug(ustae) (centuria) Mari

Property of Gaius Didius Secundus, soldier of the Legio II Augusta, in the centuria of Marius

As you can see, the word "centuria" is assumed but not written. The inscription literally says "of C Didius Secundus sold leg II Aug, of Marius"