Within the TFverse it’s Swindle who classically handles the element of trading with other parties (including/especially humans). So I was thinking he would have made more sense to be the SLS-AMG Benz rather than Soundwave. I mean, Swindle would be more fitting when you consider his past history: in a G1 episode he sold off his Combaticon partner’s components to a junk dealer; in ANIMATED he assembled a gang of human criminals to do his bidding; and in the IDW comics he is responsible for having supplied an Autobot-human taskforce with machinery (awesome irony there). This would have made him perfect and more suitable for DOTM: he would have been the Decepticon-human liaison, running a secret but brisk trade within a human market, providing businessmen with priceless tech in exchange for a few favours (for the Decepticons, more or less). In terms of alt modes, I’d see him picking the Mercedes as it would reflect his “moneybags” nature (as in making/hoarding a lot of money). I also believe Soundwave to be less ostentatious in character than to use a brand spanking new automobile as a new altmode, especially when his old satellite mode allows for more stealth. Swindle is more likely to tolerate having human partners, having established a business relationship with them; I just can’t see Soundwave taking orders from anyone except Megatron, or tolerating anyone outside of his own minions and Megatron. In fact I don’t see Soundwave at all being as he was in DOTM, I thought he was more menacing and formidable in ROTF. But if Swindle was the Mercedes, what would happen to Soundwave? Just keep him in the shadows, watching the battle take place from orbit, and keep him for a few critical scenes: deploying Laserbeak to kill them all, and perhaps around the end have him swoop down and grab Megatron’s still-warm parts and fly off into space to reconnect them. Dun dun dunn, it’s not over, here comes part 4!