Aqueous ink printer innovation is slowing down as a plateau has been reached. I'd personally like to see a gloss optimizer and more grey inks in aqueous inksets. The inkset in the 13" Canon Pro-1 is pretty awesome and large format inksets can't match it yet. Solvent innovation is the exciting area right now and more and more manufacturers (like Epson and HP) are choosing to focus their efforts on catering to high volume production environments. From what I've heard, they will not be catering to the home office, low volume, "photo" enthusiast like they have in the past. Publicly they'll say the opposite of course.
What most people in the know can't say (but I still can) is that Epson is working on a new head for both the x900 series and the Surecolor printers (which appear to share the same heads) that they'd like to get out ASAP (its been in the works for a while already). How that will come to market I don't know. Onsite affordable head replacement would be nice but a new printer designation might be another approach they may choose to take. We'll see. That part I don't know.
There are still a few things that could be done easily with aqueous inks, that wouldn't require any new technology - just a few more parts in a printer, which should be no problem for those machines aimed at high-end fine-art printing, like the 9900.
1. Printing on thicker media. It'd be nice to be able to print on thick, custom-made paper, or coated aluminium panel, without having to modify the printer for a 5mm clearance. All it requires is a raised print head and a straight paper path. Nothing too difficult there.
2. More inks. Six or seven black inks, plus a regular colour set, would give much better tonality, particularly where the tonality is in the luminance channel rather than the A or B chroma channels. Again, this doesn't require new technology - just three or four extra print heads.
3. A new yellow pigment. The one that Epson uses is known to be unstable, relative to the others. Stable yellow pigments are nothing new - all they need to do is grind up a different yellow pigment and encapsulate it, just like with any other pigment.
4. New pigments based on inert nanoparticles of varying size. These are already being used in paint, among other things. You can produce any colour in the spectrum with a carbon or gold nanoparticle of the right size, including different tones of black. It's a quantum effect of the particle's size, rather than a property of the pigment's chemical structure. These can be encapsulated in the same way as current pigments, are no more difficult to produce (just grind the solid to the right size) and won't fade at all. This would be a bit more of an investment than the others, since it would require a new line of inks, but the same pigments could also be used in solvent-based and UV printers (just suspended in a different solvent or monomer mix).
5. Pre- and post-heating elements for inks and paper. Faster drying = better results, whether on coated or uncoated papers. It would also allow up the printer to be used for both solvent and aqueous inks, opening up solvent printing to the home and small-studio markets.