I think he's actually the best cobbles rider of the GC riders (along with Nibali), because he's a little bit heavier than most and due to his mtb background. Don't forget, he was towing Nibali most of the time during the infamous cobbles stage in 2014. He also managed nr 25 or something in his maiden and only RVV participation - not that it says too much about his ability on the flat pavés in france.
But obviously, he's very limited in his options, because apart from the cobbles stage, he can only look to gain time on the other outsiders / top 5 contenders in the mountains, and I generally think the level in the mountains is going to be quite even this year, so maybe there won't be that much time to gain there for a man like Fuglsang.
A top 5 should be achievable. It's hard not to think that he can at least improve on his 7th place finish from 2013. The only "luck" he had there was Valverde being caught out on the echelon stage. Other than that his result back in 2013 reflected his strength very well. I think he's a much better rider now (as does most people), and he has a team around him too. On paper he's among the 10 biggest favourites for the tour, probably in the lower part of that top 10, but given the likelyhood that some of the bigger names will either drop out or fail at some point, it's not all that farfetched to think that Jakob can reach a top 5.