when i encounter a forgotten beast theres 2, maybe 3, ways i deal with it, but the majority is the first way. For the majority, I seal off my cavern level entrance with a bridge and open up a baited cave-in trap that captures the beast 90% of the time (the 10% being when my dwarves are too busy to pull a lever in the central control room before the beast destroys the door, which is about 1-2 days or so). And i then collapse a ceiling on it and kill it with no problem; as the cave-in doesnt even cause a blood-splatter to appear for the most part, just a corpse.
A smaller number of the beasts i encounter are lured into another type of trap, one where i actually have to use some effort in marks dwarves to kill it. These beasts are usually considered safe to engage behind fortifications, but still too deadly to engage in melee (without casualties).
There is a very, very small number of beasts that i actually get to lure into my arena and send melee dwarves to engage because the vast majority of beasts have syndromes that at the very least cripples half of your well-trained well-armored squad such that they are unusable. At the worst these syndromes is the end of your fortress as they tend to linger in splattered blood and extract.
So, i was wondering, can we have more forgotten beasts in the future that arent so deadly in a malignant way? and instead just as deadly upfront: powerful, savage beasts, more like megabeasts; that retain their trapavoid. And instead of having abilities that inflict lasting consequences on your fort, they come with strong physical abilities and skills. A forgotten beast may have higher dodge, higher speed, and destroy buildings at a faster rate, quickly overwhelming and charging past preliminary defenses; but will not have fort-wiping syndromes like deadly blood or abilities like deadly dust.