I'd just say to people to take it as slowly as you have the time to do - and can afford with stop-offs! We went 2 totally different ways, Trainer, - both great in their own way. Cheaper on our route down due to the countries we visited. Non-bracketed are places we visited, others where we stayed.
Route down - Reims, (Metz), Heidelberg, (Wurzburg, Nuremburg), Munich, (Berchtesgarden National Park and Konigsee), Salzburg, (Werfen, Villach), Lake Bled, (Lake Bohinj), Ljubiana, Zagreb, (Plitvice), Sremski Karlovci, (Novi Sad), Belgrade, Sofia, Plovdiv, Burgas, Sozopol (Nessebar, Malko Tarnovo), (Istanbul), Bursa, Kusadasi, (Milas), Bodrum peninsula, Fethiye. This took us 29 days so we could enjoy it.
Route back - Marmaris Peninsula, Izmir, (Pergamom, Troy, Cannakkale), Eceabat, (Gallipoli), Alexandropoli, (Nestos National Park, Xanthi), Thessaloniki, (Vergina, Pozar, Veria, Meteora), Parga, (Igoumenitsa), then overnight crossing to Brindisiin the south of Italy. Can go higher up to Bari or Ancona but lots of great stuff further south!
(Brindisi, Lecce, Locorotundo), Albarobello, (Monopoli), Matera, (Trani, Peschici), Parco Gargano/Monte Sant'Angelo, (Sulmona, Pescasseroli), Parco Nazionale d'Abruzzo, (Scanno, Norcia), Spoleto, (Assisi), Gubbio, (Urbino, Ravenna), then we stayed with friends in the hills above Bologna for 3 nights. (Bologna, Maranello, Modena), Como, (Lugano, Locarno), Lucerne, villages in the Alsace staying in Colmar for 3 days, Strasbourg, (Savernne), Lille, (Calais), home! We had planned on Luxumbourg too but ran out of time as 90 days insurance - home, car and holiday - expire then without difficulty. This took us 33 days.
We saw magnificent castles and churches, cities and countryside, wild wolves and flamingos, high hills and mountains, amazing waterfalls and fabulous remote villages, monasteries teetering on rock pinnacles, the graves of Alexander the Great's father and son, complete with artefacts, the largest ice caves in Europe, trulli dwellings and cave houses, as well as other amazing architecture and mosaics (particularly in Italy), ancient Roman ruins and seaside towns. We wine and champagne-tasted, enjoyed some varied and tasty food and drink, shopped at totally different destinations, went to the Ferrari Factory and museum, walked in spectacular countryside, took boat trips on lakes, loads of cable cars, watched pasta being made and a falconry display above a Medieval Austrian Castle high in the mountains. Just a few of our experiences! If you can do something similar you will have a fabulous time, but, personally I see little point in belting there or back in about 4 days!