I would avoid using the futuristic weapons in the DMG; they're vastly overpowered, and are meant to be treated as super-rare, almost artifact-level mojo. That seems very different than the ubiquitous blasters of planetary romance. If I were creating stuff for this genre, I'd just reskin crossbows, but have them deal radiant, fire or sonic damage, and make them all simple weapons (yes, even the hand crossbow -- it becomes the blaster pistol, the other two are light and heavy rifles). This produces a game where classes like Cleric and Sorcerer and Rogue will prefer blasters, but Fighters can dish more damage overall with melee weapons. There may also be practical reasons for melee weapons, like they may be quieter, more easily concealed, and not run out of ammo or jam or become "drained of charge" or whatever.
Vehicles! Weird vehicles, like rocket-sleds, jet-packs, and airships, seem like a common part of the genre. I'd definitely include rules for some of those. Wireless communication was a thing, but it's the kind of genre where you might find couriers and messengers to still be used quite heavily (maybe the wireless is short-range and low-quality). Exotic mounts are also cool, like pterodactyls and giant lizards or insects. Matching these crazy conveyances should be terrain that is just as extreme, like sheer cliffs a mile high, or subterranean swamps, or an ocean with a storm that never stops (but the eye of the storm is a peaceful island paradise!). In a way, you can be more creative with that kind of stuff in planetary romance than you can in standard fantasy, because you can use retro-future technology to explain away any difficulties ("What do the people who live on the cliffside eat?" "...Hydroponically grown plants and miniature animals.") Somehow handwaving that stuff as "magic" always rubs me the wrong way, but hand-waving it as corny sci-fi seems ok.
