Road movies are?a long standing tradition as they, in my mind, emulate the inner journeys of the character in physical form. They provide a sense of adventure via movement, in almost a synesthetic?like cross over between the external sensation of moving and the internal sense of learning. The characters, and by proxy the plot, feel better paced as they move forward not just in a literal sense, but also as part of a developmental process.
This works best when a journey has at least one logically compelling reason for the characters to go from A to B. The subplots that arise, the complications that come about and ultimately the inner journeys are elevated as a result of being imbued with the forward momentum of the compelling outer physical journey.
However, in this concept I fail to see what the compelling aspect i. It was written in the will of a friend - this isn't enough.
I think it's safe to say you want to write a road trip movie across Europe.
Working back from this assumption, I can suggest a few other reasons to send a couple on a road trip:
1) One of them has a few months to live, but they discover a child he or she has that they never met before. As they are afraid of flying or can't afford to fly, they head out on the road to meet the child before it's too late.
2) One of their parents left one of them when they were children, now the parent has a week to live so they take off on a trip to meet them before they die.
3) One of them is given a condition in a last will and testament - in order?to get the family castle or fortune,?they must visit all the villages their father had fought in during WW2 accross Europe, and apologies to the residents in his name.
Point is, I believe that the possibilities for the premise have not been explored enough yet and there are many you could use.