It's intended for artists, designers, hobbyists and anyone interested in creating interactive objects or environments.
For this market, a product has to be "right-sized." The Duemilanove with m168 was about perfect for a lot of things; the m328 versions added additional memory space "for free", so that was OK too.
The next question is "what else will people pay for?" Arduino MEGA? Networking? Depressingly overpriced in the Arduino arena, IMO. But a lot of those "art" and "design" projects seem to have a weird budget model (compared to "hobbyist" or "commercial" anyway.)
After that, perhaps "how much can be improved at the same price points?" I haven't been inclined to do much with MEGA or Due, but I have some Teensy3 modules, ChipKits, and m1284 boards. They're swell, I think. Arduino Zero might have potential.
Things like the RPi, BeagleBone, ST, NXP, and TI ARM boards have always bothered me. As a hobbyist, they're OK. But my theoretical "goal" is small scale production of custom electronics (say, 100 to 1000 boards?) With a mega328-sized design, I can easily plunk down the "meat" of an arduino on my own board design, assemble them by hand, and be done. For the small pitch parts on the "more powerful" designs, I don't think that there is any way I can duplicate the cost of the sold boards with my own board, by the time I do the 4+ layer PCB and contract assembly. Maybe I shouldn't care, and using vendor-subsidized
modules with questionable product lifetimes and licensing issues is the way to go. 
I watched with some amusement and horror as linux system administration issues on RPi and BB replace the programming and electronics issues people have with Arduino/etc.
I'd like to see an OS that is closer to Bare Metal for BB and RPi. Or an OS for Teensy/NXP/ST/TI that is better than the current bare metal, without sacrificing the "ease of use" of the Arduino libraries. But I don't think that either one exists yet.