Onderzeeboot: that's a cool card but I feel like it's one BR effect instead of a (throwback undercosted) Black Effect + a Red Effect. I think +2/0 and Haste might be the right choice for the red half of the card, Hero probably already has suitable targets for that since there's a party in play, is there a different effect we could give the other side?
Looted Trinkets
4BR
Instant
costs
less for each creature in your Party.
Target creature gains +3/+0 and Haste until end of turn. (Reckless Charge)
Target creature gains -1/-1 and Fear until end of turn. (Cursed Flesh)
Anything that is symmetrical removal of resources (i.e. Wrath) tends to be better when you're behind on that resource - the player who is ahead on board is unlikely to want to go back to parity. (Discard might be the one exception? is "Each Player Discards their hand" something you'd want to do when you were behind?)
Anything that benefits blocking is probably better when you are behind. (Reach is the Marquee mechanic here, does nothing if the creature is attacking.) I think Bushido also benefits being behind, outside of a set where 50% of the creatures have it (then it's just 'being better than creatures that don't have it')
Mechanical Space that could probably be explored: "If you lost life this turn" on instant speed effects (reverse spectacle), "if your opponent gained life" (Punishing Fires but as a mechanic), "if you have fewer X" (land tax but as a mechanic, etc.), something like "if your opponent controls a creature with the greatest power..." (reverse ferocious kind of), "if you have fewer cards in hand" (this maybe benefits going hellbent which isn't necessarily behind?). "if you have fewer cards in your library" (maybe benefits mostly self-mill?), "if a permanent you control left the battlefield last turn" (self-sacrifice?)
From Behind 2UWR
Sorcery - Trap
This costs
if you have fewer cards in hand and fewer permanents in play than target opponent. (lands are permanents)
Choose 3: you may choose the same effect multiple times.
* Draw a Card.
* Target Permanent you don't control phases out.
* Deal 2 Damage to each creature and planeswalker.
This is maybe broken? definitely a better card if you're behind on resources though.
edit: let's push the red effect harder.