ER Ironhide was the wrong template for this design. Something more like Origins Wheeljack or even better something like a Battle Sled style Ironhide would be better.
Weirdly, while I kind of like the Origins Wheeljack vibe for a different Party Wallop, I don't really want it for this. Partly because the "toys all ride inside" was such a thing for the original Party Wagon toy, and while I remember a lot of shots and scenes of the gang inside the Mystery Machine, it's definitely not what comes to mind for me first -- it's the looping-background shots of it driving along. (I never had a single piece of Scooby merch, but I'm sure there were Mystery Machines that could also carry the figures.) Nevertheless, if you're going to include anyone, I agree that them being able to ride this would enhance it a LOT.
It really does in some ways feel like the plans or concepts for a more robust offering got culled down one by one until this was what was left. Imagine at least a Godbomber style situation where the front of the van opens to seat two or three Titan Masters, and the fourth and Automutt, er, store in the back or something. Then the cockpit/storage went, the Titan Masters got simplified down to just plug-in heads, and so on. Automutt survived because the whole point is Scooby Doo.