Id like to see the ARES or Oden put into MP, maybe to be used one at a time, like the mothership? And as far as the warhound goes... I think that the warhound looks like an SCV, so why not make it to where SCVs can be upgraded/built into the warhound after some time and research? Maybe... needing an additional SCV to make the construction possible (like contructing a building) and at least one command center and a factory/armory? It sounds weird having to build single units like that, but thats what makes terrans more unique IMO.
Those units would never be put into the game. Units with 1500HP are way too many to ever be played. SCV's will never be able to be upgraded, it would give you too much of an advantage over the other races. A mining unit that can be turned into a combat one? as soon as your base came under attack you'd simply turn your SCV's into a warhound, whilst trying to buy yourself enough time for them to transform. There's a reason that single player units are in single player: They are simply too overpowered.