When Arenanet asked the community for trick/trap suggestions, many years ago, they made caveats that meant they got barely any responses.
One such caveat was the stipulation that the trick/trap can't do damage. Ridiculous.
With that stipulation firmly defenestrated, we can move on and come up with the tricks/traps we should have had.
Proximity Barrel (Explosive)
My suggestion is to begin with the Proximity Barrel (Explosive). You can move on to "fire pool (burning oil)" barrels or "cow effect" barrels or whatever later.
The one property that Barrels should always have is counterplay. So the trap should be visible to the enemy. If enemies shoot it from range, it should explode.
Obviously, this specific barrel should explode, doing approximately 50% of HP damage to all enemies in the area of effect. There must be no target cap at all. Not 25, not 50, none at all. Map blobs should fear the proximity barrel and make special efforts to counter it.
Nice to haves
I'd like the barrel to do knockback.
I'd like the barrel to blow up other barrels, giving players access to hilarious chain reactions. These types of mechanics will lead to very popular social media videos.
Points of Balance
If the barrel needs nerfing;
the amount of supply required to build the barrel can be adjusted,
the AoE can be quite small,
the damage can be adjusted,
the knockback can be reduced/changed to knockdown/removed.
Heh, ye. What might have been eh? If only Anet had the willpower ... /sigh ... I guess another company will have to get the glory instead.