Unknown Soldier

Try groups like Amorphis and Evergrey, these are groups with a lot of prog rock influences and are not really heavy as such and go more for the melodic approach. Opeth are another obvious choice with their traditional prog influences but the earlier stuff falls into more progressive death metal territory! Another obvious choice would be Dream Theater but I`m guessing you probably know them. I also recommend Fates Warning as well, they have a lot of good albums. Also, if you like the symphonic influence then try Symphony X as well.

All of these groups are quite different, meaning that somebody that likes one may not necessarily like the other, so its a case of listen and see. They`re all classified as prog metal which tends to cover a whole host of sounds. With the exception of some of Opeth`s stuff, none are are really heavy and focus more on melody.