The death of megabeasts exists in-game for a reason. It's part of the story that DF is built to tell. There's a logical progression of the ages that
exists as part of the game's coding.
In much the same way as Narnia or Middle Earth, (and I'll remind you, DF draws a LOT of inspiriation, especially the early stuff like the ages, from Lord of the Rings...) magic and the mythic are supposed to be gradually fading out of the world as it ages. That's kind of
why dragons and rocs don't breed in worldgen in spite of being able to do so in play. Catching The Last Of The Dragons is more interesting than catching dragon hatchling #25126. Catching the last female dragon, and pulling out your male dragon such that you can breed them to keep this majestic, if terrible, beast still in existence such that magic will not die from this world yet is also an epic story waiting to be written.
If you want more megabeasts, there's two simple solutions for you: Crank up the megabeast population in worldgen, and stop worldgen sooner, when less of them have had a chance to die. If you are starting out with 200 megabeasts, and stop worldgen at year 100, I can guarantee you'll not be running out anytime soon. (And for that matter, you might find elves or some other races nearly extinct if the megabeasts rampage hard enough...)
That doesn't even take modding, that's just hitting "advanced worldgen" and going to megabeasts and punching in a bigger number.
Likewise, megabeasts already hibernate, functionally. This is mostly what Forgotten Beasts are doing all the time.
Now, making, say, hibernation cycles such that bears, snakes, and frogs hibernate (and maybe some Titans, dragons, and FBs, too for a few years, maybe,) would be a decent addition to the game, but I don't see a reason why we need to make titans even
more powerful than they already are when several of them (flying steel blob with syndrome titan, I'm looking at you!) are functionally invincible in any "fair" fight, anyway, and the ridiculously flimsy ones (mist titans that snap in half on a rabbit's nibble) wouldn't be helped by just arbitrarily upping their stats, anyway.