Grimlock would be a semi-rational choice and plausible if not for the fact that he's just a mechanical dinosaur. So this would really be a decision hinging on whether Takara thinks that an MP T-rex would appeal to the monster lovers on the market, or whether people would be turned off by an over sized elaborate dino?
Remember too that MP Grimlock would have to be faithful to the cartoon - and let's face it - that's not too exciting a dino... he's retrograde with posture and pretty lacking in detail. Sure, they could "upgrade" his dino mode, but I think that would border on making him more unrecognizable than grey MP Starscream - a risk they might not take.
So -- no...my guess is they'll shy away from that idea too...
Grimlock is the most logical MP from a culturial and profit standpoint. Japan puts out more "Japan Only" Godzilla toys a year then have ever been released over here! Grim's almost too sure a bet. He'd outsell Magnus & Starscream combined and when the dust clears probably Megatron. He's their ultimate trans-genre character this side of all the crap they slap Hello Kitty! stickers on. And it wouldn't be hard to engineer the T-Rex mode to stand G1 upright or JP (Jurassic Park ) bent over. Just needs the original Monster feel he had in Season 1 over the "Danial's Pet" vibe he had in Season 3.
Plus a BONUS: MP Wheelie to sit on his nose!
Soundwave - it's the scale thing. Making him to scale with Megatron would make him overtly big as a tape deck. Now, you can do that with the Walther P-38 because it's not THAT much of a stretch, but I just don't think it'll be credible with Soundwave. Revamping him into a bigger sound system would also miss the point of remaining faithful to the original. Takara's plans for Soundwave are obvious - MP3 Soundwave is the direction they're taking the character - not MP.
Do you remember how big cassettes really were? I got a Soundwave Cassette Player to 'bash into a MP but it's too big! A real cassette is the size of MP Primes chest! The fact is he wouldn't be too large, he was too scaled down in the 80's! The only other real option would be to scale him for the mini cassette recorder tapes but they were smaller than Laserbeak in alt mode. Thus, the dream of a working, G1 detailed Soundwave dies now with us... but they'll try it anyway for the sake of PROFIT & the BETTERMENT OF JAPAN!!! Go JAPANESE WORK ETHIC!!!
Oh, and the 'Cons are outnumbered 3/2 right now so the only way they'd do a 'Bot next for sure is if they do both Thundercracker and Skywarp. And we all know they won't do the same mold back to back again. Magnus (he's not Ultra w/o the armor to me) was just testing the waters to see if demand was high enough to justify the continuation of the line.
4-EVER