Since some time after I made this thread, I've been contemplating and eventually decided to do an unusual decision regarding the mod's direction, that I hope won't be a disliked one. Lately, I've been re-playing Mario & Luigi: Bowser's Inside Story (playing the 3DS remake for the first time), and I really like many of its gameplay elements (alterning between a single large, powerful being in the overworld, and weaker peeps inside said being's body to help them from inside) to the point I get rather annoyed with how nobody else did anything like it. That game already was a major inspiration for the basic concept of the mod I talked about, namely the idea of being able to explore one's body as if it were kind of a dungeon - but I feel like simply teleporting inside an NPC and exploring their insides would get boring after a while, since most Starbound species have normal digestive systems with nothing much to do within them - additionally, it's likely impossible to be able to seize control of another NPC. So I was thinking of adding another feature in addition to that - having some sort of special "companion" (actually a "vehicle" skinned to look like an alien creature) where you're able to have it swallow your character whole to "teleport" you into inside their body, and while you're inside them, you're able to swap control between the companion in the overworld, and your character inside your companion's body - much like M&L handles the player swapping between Bowser in the overworld, and the Bros. inside his body.
This companion would have unique abilities, but many of them would start out either locked or fairly weak, which would require the player to explore certain dungeons set in specific parts of their body to be able to reach a specific part of their body that, when activated, unlocks a new ability and/or makes an existing one stronger, so the companion is able to keep up with planets of higher tiers. There will be an in-lore explanation for this, all I'll say is that it involves repairing broken bioaugmentation enhancement implants inside your companion's body, so that said enhancements start actually working and grant new abilities. Moreover, when playing as this companion, they would have the ability to eat other enemy NPCs whole (so long as they're non-bosses, obviously) so that, in case the player is currently stronger than their companion, they're able to fight said enemies from inside the companion's body. This also doubles as letting the player play as a pred creature in addition to being a symbiotic prey, which would further enhance the appeal of this mod to Starbound players who aren't exclusively prey, but don't wish to play multiplayer.
To compensate for the gimmicky, convoluted and possibly unnecessary gameplay twist, playing as the companion would be a different experience than regularly playing as the player character: they're faster, stronger, and more resistant to damage in average (but not to overpowered extents), but in addition to not being able to have active crew members (they'd have to be exclusive to the player, and when seizing control of your companion whilst having crew members following you, they would teleport inside the companion's body to accompany the player character), likely would have some other limitations or gameplay elements to balance out their strength. The main obvious obstacle is that they would lack the finesse of the player character, such as the Sphere Ball tech, and would lack long-range attacks until the lategame. Another possible limitation could be them having a "hunger" meter that makes them less and less combat-effective if they've gone a long time without having swallowed an enemy, thus requiring the player to still be in a decent enough shape and still have to fight overworld enemies from time to time, as opposed to having to rely on your companion to defeat all enemies.
Obviously, this would not replace the intended core functionality of travelling inside NPCs' bodies, but it is a feature I've been wanting to work on for a while now, as it would make the mod far more unique while appealing to both prey and pred players. Plus, the newest update of SSVM already provides somewhat of a basic functionality for the "vehicle" part of the mod, specifically the work-in-progress dinosuit. Even then, however, it's still a rather ambitious project to undertake, and as much as I want to see something like it become reality, I do worry about whether I'd be biting off more than what I can chew.