Well, I am trying to analyze the optimal travel time between the islands based on the quests, but it's proving to be no easy task. For example, so far it seems that Rehau must be done fist, because it gives out quests to verona and then elona (DeGosh's papers). Elona gives a quest to Shatterra (Liu's soul), so it must be completed second, and Shatterra must be completed last, then back to Elona, for all it seems, since a lot of quests from various areas end there (klisan's soul, the tools for the fallen alien being, etc.).
In addition to that, a lot of side quests that finally end in verona should be completed when you collect all stones and just before you speak to the turtle Theana.
Also, it is imperative to buy a boat in Montero. Travel time from montero to the adjacent islands (like Tekron, Rehau, etc.) is way lower than from verona, and besides, the travel time from verona to Montero through the bridge is 5-10 mins, based on whether you are able to fly or not
Well... and travel times can also be cut by using the 2x/4x sacrifice/1x time back trick, allowing no extra time to get back to restock the troops.
Any other ideas will be appreciated