Howsabout "facccioscuro" from Paolo Bacigalupi's novel, Navola.
A poker face; an illusion cantrip that conceals whatever one is feeling, hides tells, and can be extended to body language and posture if cast at first level.
Unless there is already a pretty good illusion that manages it and I've forgotten.
How about it, indeed. I'm not going to be brewing up anything new now: all of the remaining regular posts are queued and I'm on holiday for the rest of the year. All I can offer you is some thoughts on the concept.
Firstly, I'd point out that what a person is feeling is almost never referenced in the game mechanics. The only two interactions I can think of are the detect thoughts spell and the sprite's Heart Sight ability. Such things are rarely relevant to D&D gameplay; as a DM I usually just handwave it.
The other elements you mention; tells & cold-reading; are rolled up in the Insight skill. The game wants to make it so that the person trying to read your tells is the active party, making the roll. I.e., if their Insight is good enough, they can read your true intent. Unless of course, the DM decides the situation calls for an opposed check, in which case I'd generally expect Performance or Deception to be used in opposition to someone's Insight. This is an annoying grey area, created by D&D's half-hearted attempt to make 'social interaction' into a 'pillar of play', without offering enough rules to fully support it as such.
Well anyway. I'd see two ways of implementing an effect like this 'faccioscuro': a weaker "creatures have disadvantage on Wisdom (Insight) checks made to read your intentions via body language" and a stronger "for the duration, other creatures cannot read your intentions by observing your body language". I'd note that the latter of these is already provided by the Ring of Mind Shielding (an uncommon item). The only spell that covers this ground in the core books is mind blank (an 8th-level spell) - though that one does a lot of other things besides just making you unreadable - so I would say there is room for a lower-level spell that does just this. It's a bit similar to nondescript, a spell from previous editions that I converted to 5e years ago, but it's distinct enough to be a separate spell.
So the effect is weaker than an uncommon item (the ring also protects against telepathy, which a visual illusion wouldn't, if I was writing it), especially the weaker 'disadvantage' version. But is it cantrip material? Taking it as a cantrip basically just turns it into a passive effect that means no one will ever bother trying to sense your motives by observation alone. That doesn't feel super exciting, and I'm... not sure it'd be worth taking over the cantrips that already exist. Given how little impact this is going to have on gameplay, it'd be in the realm of ribbons that I'd be willing to just let a player have for free, maybe as a quest reward, or part of a custom background, or something.
Alternatively, the stronger 'immunity' version probably works as a 1st-level spell with maybe a 10-minute duration. I think I prefer it as a spell, because then you need to be selective about when and where you use it. It also fits with my theory of 'first level spells often replicate effects that you could also achieve through mundane means' (in this case, a good Performance check to maintain a poker face).
If I was going to write this up, I suppose the template would be protection from evil and good. The aim would be to make this into a 'social' version of that. It'd still be pretty niche, but on-brand for bards and cheap enough for wizards to consider scribing it (could also substitute one of the domain spells for trickery clerics or be a once/day racial spell for a homebrew race), so maybe worth adding to the game.
