I'd like to see the next evolution of the Land Raider (and variants) that has the treads removed and grav engines installed. You could have a nice bit of fluff written for it. Perhaps Guilliman, as a secret thanks for Helbrecht agreeing to cross the rubicon, tasks the Admech to modify Land Raiders and bring them in line with the new marine equipment. This would also fall in line with the direction GW has taken with phasing out the old marine models while giving us (not just Black Templars, but all marines) a new, expensive kit to buy. It would be glorious to see a grav tank carrying 12 primaris marines wearing terminator plate or gravis armor or 16 regular primaris!