Who said Motorola has to make it?
Any manufacturer could build a DMR radio for use on the amateur bands.
True, Vertex could make it, but other than Motorola and Vertex, no one else makes true 100% compatible MotoTrbo in all it's glory. Motorola, as they like to do, add "enhancements" that make their equipment not always 100% cross compatible with other radios that follow the "standards". They are actually pretty famous for this in the P25 world.
I don't see Motorola licensing true 100% compatible MotoTrbo, (The OP asked about MotoTrbo, not DMR), anytime soon.
If you are talking just DMR, then yes, I imagine anyone that really wanted to would make one, but we are really getting down to semantics now.
But, I really doubt that adding a third digital mode to VHF and UHF amateur radio will be met with enthusiasm, other than by the hard core Motorola fanboys. Yaesu has theirs, Icom has theirs. People that really decide they want to play DMR or full MotoTrbo on the ham bands can just buy one of the existing LMR radios that will do this mode. I'd be surprised if there is any business model that would do this, but I'd love to be proven wrong. Who knows, maybe Kenwood or Alinco will roll out DMR as a competitor.
The original question was which one do we think will come out first? Scanner. Many of the scanners on the market already have the AMBE vocoder in them, so making them run DMR wouldn't be that big a deal. I don't know of any amateur radios that have AMBE vocoders in them. If there are, I'd be interested in hearing about it.