Finally beat Battle Network 6, so I'm finally done with the Battle Network series bar Battle Chip Challenge though including Network Transmission. It's funny but after playing them all I can't really rank them beyond BN5 DS being my favorite and that Network Transmission is underrated. They're all just so similar.
Still, I definitely get the feeling that the series would benefit the most from a hardware upgrade. Larger/more areas (beyond 4-5 key areas), CD quality music, online battles, etc. Would
especially love to see redesigns of the MM9/10 and unused RMs (Ex. Crash, Gemini). Though I would hope they don't limit it to versions again and put those extra elements and development resources to better use.
Honestly I could easily see Capcom create an MMO (sorry Rockman Online) out of the series. Creating your own Navi and customizing it with various skills, attacks, elements (I think there's around 11 now), would be pretty awesome.
IMO:
What Would Actually Sell: X9 (MM11)
What MM Fans Want: Legends 3
What Would Benefit From an Upgrade: BN7
Story Deserves Finishing: ZX3