These are all fairly budgeted digital games so far;
Beach Buggy Racing... Basically a Mario Kart that isn't actually that bad, regardless of a name so generic it's instinctual to gloss over entirely.
Renegade Riptide GP... Future boats in future world setting, there's 2 of these games on the store and it's the sequel that contains the best buck for bang, but the sequel isn't the one with a '2' in its' title, so don't be fooled!
Trackmania Turbo... There's a demo for that available, that's unfortunately the extent of my experience there, but it may suit your needs.
These ones are generally considered to be good, but I cannot personally vouch for without trying them myself;
Mantis Burn Racing... Isometric racing with 'rpg elements.' There's a couple of similar titles in this genre available.
Wipeout: Omega collection... I assume most people already know what Wipeout is. Unless its' popularity was confined to solely the UK. Well this is supposed to be the best of that, distilled into one polished package.
Blaze Rush... Am not sure how to describe this isometric insanity, but it looks pretty damn fun and I should really get around to trying it soon.
There's probably more, but those are basically the digital-only downloads that seem to provide close to what you desire. Weird that offline splitscreen is so often absent in bigger budget titles.