I have tried to workout a solution with minimum demolition and construction along with keeping minimum length of the floor pipes from the new bathroom. And solution for pipes would be next to the columns in soggiorno location a believe it have some pipes in it as show in your plan image so kindly provided by you.
I have placed the guest room as far as possible from the master bedroom for privacy/sound privacy and keeping it next to living room.
Dinning area is next to kitchen visually range for better communication.
The existing TOILET have been (no change in layout) converted into common toilet for the guest room and other guests.(as you mentioned that you dn't need any tube in guest toilet .
Ripostiglio is converted into laundry room.
GOOD LUCK FOR YOUR PROJECT
Regards
BLACKTREE
P.S:-for dimensions check dwg and all dimensions are in CM
I have also uploaded two living room render images looking toward south windows.
