What car are you doing this in?
I'm trying to work out the next road trip in my Gran Cabrio - we've done the Italian Lakes, Tuscany, Corsica and Spain and we've got a complete circumnavigation of the coast of Spain and Portugal booked starting in May this year (6 weeks for that one)
Puglia is definitely one option but I just can't shake the desire to drive to Crete. The other option is Norway / Sweden / Finland, but that seems to be more of an off-roader destination than GT.
We did Timmelsjoch a couple of years ago - personally I wasn't super-impressed - it was fine, but the drive from the end of the toll road to Salo in Lake Garda was really tedious. We had GrossGlocker on the list for last year and even had overnight accommodation booked at the top. It was mid may and it was completely closed so we had to re-plan and went to Venice instead.