I don't know if it would be the ultimate, but I'd like to see is a full range, crossoverless plasma speaker, hopefully with direct drive electronics. Several years ago Nelson Pass developed a full range plasma speaker, but few people had a chance to hear the pair he built before (I think) he decided to dismantle them because they were too dangerous. Anyone who ever heard the infamous Hill Plasmatronics 25 years or so ago knows what I'm talkin' about. That tweeter!
I heard it said years ago that the "perfect" (note the quotes) audio component has three characteristics: very expensive, dangerous, and not available. The Hill Plasmatronics were sort of available but surely met the other qualifications. The built in tube amps were pretty unreliable, so I understand, although that's surely not a desirable quality (been there, done that!).
Among "real world", *available* speakers today, I like full range single driver speakers a lot. With driver integration not being a concern, they stand a better chance of recreating the experience of listening to live music. Along with that, dispersion and sound radiating geometry play important roles in creating the illusion of live music including reflected sound, and in my experience few box speakers are capable of giving that natural sound quality.
Brian