Probably the only two I would like to see merge would be Roland and KORG. They kind of operate the same way when it comes to customer support, and the technology between the two is very different.
From a design point of view, I bet they could come up with some very futuristic gear.
Ketron are best left alone, as someone like Yamaha would only destroy them with their business practices of sell now, screw you later. While Ketron could do with some design help, their keyboards would easily give KORG a run for their money on quality of sounds and styles. It would be hard to choose actually.
Yamaha have always done their own thing, and generally should continue doing more of the same. Nothing really new or ground breaking. Just more of the same only better.
Technics, as far as I know. KORG hired a lot of those guys and bought out a lot of their style libraries. Most of which can be downloaded for free off of korgpa.com
Regards.
James.