new floor structure in the old building ground floor

Attached is the plan (which, however, has little to do with reality) [...] the plank floor lies on wooden beams that rest on a simply flame-treated concrete slab, with slag in between.
Thanks again for the help!

You can certainly get quite a bit of help - depending on how much of the rest of the plans you show. Especially regarding the floor you specifically asked about here, the sectional drawing is unfortunately not very informative, but otherwise quite a lot can be read from it, and from the whole set of plans, of course, even more (plans make the most sense when they are understood "mutually"). Consider how much advice you want to receive - in my experience, questioners also benefit from suggestions about aspects for which they initially had no open questions; therefore I would advise you to also discuss such aspects where you already feel "confident" that you have found the "right" / best / most appropriate way. Tending to say, what you have shown so far suggests that a modernization is worthwhile, but will also involve some work (keyword: energy-efficient refurbishment). You should describe the deviation from reality in more detail - I suspect you primarily mean parts that have since been altered or supplemented, rather than original deviations between plan and execution. Among the properties I have encountered here, yours is likely most comparable to this one from 1954: , although to me the exterior walls and ceiling/roof truss construction of your property appear more favorable.

Since both the basement and living areas are all dry, I have thought about a complete renovation of the house. As far as the heating is concerned, I am still not quite sure and would like to be advised by an energy consultant. Also planned are an ETICS, basement ceiling insulation, new windows, top floor ceiling insulation with an unfinished attic, a new roof, all installations and electrical work. On the upper floor, I will leave the cinder and perhaps only fill it with dry bulk material, if necessary double up the beams if they are too uneven, dry screed; from the current perspective, I would then not need to set any overlaps higher and could achieve a good result with "little" effort. Since there is a concrete slab in the non-basement area on the ground floor as well, I would work here with bound bulk material, again dry screed. Underfloor heating is out of the question for me due to the wooden beam ceiling and the effort involved in the floor construction. Please provide alternative suggestions; I am a craftsman but not in construction.

Do you not intend to extend / convert / change interior walls? Based on the drawings shown so far, I am still unsure to what extent the roof structure can bear insulation that complies with the current retrofit regulations. An additional wall construction on the outside seems feasible to me here. Have you already read the linked example?