MP Skyfire/Jetfire - I wish HasTak would make this, but I don't see it happening, though with the new scaled approach to the MP line he'd only be what, around the size of MP-01?
Neo-G1 Arcee - Pretty sure an MP would be physically impossible, and unfortunately it doesn't seem that a neo-G1 style figure is likely either.
Neo-G1 triple-changing Springer - I know there are rumors about a new FOC styled voyager Springer, but based on the track record of his figures I doubt it will be a triple-changer. HasTak seems to have abandoned that part of Spinger's character.
MP Galvatron - I know there are a lot of people that want this guy, but it doesn't seem terribly likely. The 'new' line of MPs seems to be focusing entirely on pre-movie G1 characters (or else I think we would have gotten at least MP Galvy by now, since MP Rodimus is oft stated to be a part of the 'new' scale).
MP fully armored Ultra Magnus - See above, also honestly it would be a pretty complex/expensive build. Magnus does seem to be getting some love from HasTak of late, though, so who knows.
The rest of the FOC Dinobots. - Grimlock always gets all the love, also there's that issue of what happened to Sludge.
MP Jazz and Bumblebee - Jazz is one of my favorite TFs ever, and a proper MP G1 VW Beetle Bumblebee would be a wonderful thing. Unfortunately, last I heard, there was still the issue with VW and Porshe licensing the alt-modes. Though HasTak did manage to get the Lambo license for Sideswipe and crew (though I do wonder if some issue therein is why there has been no mention of a US/Hasbro release of that mold).
New Scale MP Megatron? - Honestly I'd prefer Galvatron, though I would like a Megatron to stand opposite my MP-10 Prime. The problem obviously being the alt mode. I really wish there was a way to get a great G1-looking Megatron figure that believably turns into something else.