I have long abandoned my aesthetic reservations against arrows. I'm sure I can come up with something that I like. And to the degree that I can't come up with something I like, the substantial utility of representing the direction of links easily outweighs any aesthetic concerns.
The real hold up at this point is technical. A brief survey of the mapping apps of the AppStore reveal a relatively large number of apps that don't have arrows at all. However, the apps that do have arrows, all use tricks that are all fairly unsatisfying.
The difficult is that there is no trivial way to find the intersection between two paths in Cocoa (Apple's user interface library) and I suspect it doesn't exist in Cocoa because it is non-trivial in general.
This results in apps that choose from a number of tricks. For example, some apps place the arrow well short of the shape in order to guarantee that it isn't obscured by the shape. Other apps create 4 anchor points on a shape (top, bottom, left and right) that greatly limit where a line can intersect that shape, which results in some weirdly placed links.
They may also have fairly limited shape types, which is a problem for Oovium currently with it's different bubble types, but is about to get significantly more problematic going forward.
I mention all this not to argue against implementing arrows, but rather to try to explain why I haven't added this seemingly obvious feature yet and to assure you I'm fully on board with the concept; I just need to figure out how to do it right.
All that being said, I have some enhancements in mind along these lines and I have been thinking that I would re-visit the arrow issue again at the time. I realize Oovium has been a bit quiet recently, but I'm hoping that that will change before the end of the month. There should be some big releases and announcements coming soon. I will try to prioritize the arrows in the context of those announcements.