Realistic creatures are built on trade-offs.
Want a more robust metabolism that can handle more substances? You’ll need a lot more chemical pathways, more enzymes, more protection from the dangerous substances, and more overhead in general. That means less energy per pound of flesh – less speed, less strength, being slower to grow and heal, and less ability to support a brain or other special adaptions. There’s a reason why the most environmentally-tolerant organisms are bacteria.
Want to swim better? You’ll want streamlining, short limbs that produce less turbulence, skin secretions that make it easier to slip through the water, and many other adaptions – none of which are going to make it easier to get around on land.
Want to run faster? You’ll need to give up something else. Durability? Being able to carry offspring until they’re large enough to be sure of survival after birth? The time you can go between meals?
That sort of exchange – more commonly known as an “energy budget”, “developmental budget”, or “opportunity cost” (among many other names) – is why cheetahs don’t travel at three hundred miles per hour, why you don’t see creatures with claws, fangs, spurs, poison, armor plate, quills, intelligence, gills, and lungs, all at once, and why various predators can still kill turtles (which have been pursing the “armor plated” path for a very long time indeed). At some point, the gain from improving something that’s already good isn’t worth the cost. Instead, most creatures hit a balance point, and either don’t change a lot thereafter or swing back and forth over a narrow range of extremes. Such a species will remain fairly stable until something changes radically in its environment – whereupon it will either find some new tradeoffs or go extinct in the area affected.
Thus dinosaurs shouldn’t be much more dangerous than current predators (if, perhaps, somewhat more surprising due to their unfamiliarity) – unless there was a tradeoff for that. In fact, research on air trapped in amber suggests that there was one: dinosaurs may have been able to be faster, tougher, and meaner than most current animals because they were adapted to a considerably more oxygen-rich environment. That meant free energy – which could be invested somewhere else.
Occasionally – but very, VERY rarely – there may be fundamental improvements. A stronger structure for bone perhaps. A compound that carries oxygen in the blood more efficiently without other major costs. The ability to use poisonous oxygen to boost the amount of energy your tissues can produce. The ability to secrete minerals to form a hard material suitable for protection and support. The ability to reproduce without laying soft eggs in water.
That kind of thing tends to lead to whole new orders of life forms and to the sudden extinction of many older, competing forms. It also tends to be something that only happens every few tens of millions of years. In fact, after a certain point, it may not happen at all any longer, or at least not by any natural cause.
Now, most gaming worlds offer more than simple biological options. Magic, psionics, strange energy fields, “mutant powers”, exotic materials, and strange metabolisms all make their appearance. Still, there still must be tradeoffs; if there was some ideal creature design that came close to being all-powerful it should dominate the multiverse already.
Here are the rules:
1) Every organism has a budget and makes trade-offs. If you want it to grow to adulthood in ten minutes, reproduce every twenty minutes, and survive a reasonably wide range of environments while waiting for one in which it can flourish, it’s not going to do much else. You want it huge, long-lived, armor-plated, intelligent, and equipped with massive powers? It will have to pay for that. In fact, such a creature will probably HAVE to wind up reproducing and maturing very slowly, have various vulnerabilities, require special resources to live and grow, and more. That’s why variants on said creature won’t have taken over every ecological niche on the planet. Every quality you give a creature – rapid reproduction, high intelligence, armor plate, acid blood, whatever – should come out of it’s budget. Once it’s been spent, it’s time to start throwing in drawbacks. You don’t really need a formal system for this – but it’s something to keep in mind.
2) The more sophisticated the adaption – whether that means high-powered magic, a culture’s technology, psionic powers, exotic structural materials, or something else entirely – the narrower the range of natural laws, and thus worlds, in which it will work properly. This is what keeps those pesky “elder races” from taking over the multiverse. It also means creatures from very old worlds will be extremely dangerous – but that they’ll be very dependent on the local conditions they’ve evolved for. If you can just get out of their reach, you may be able to get away.
That is why one gaming group, long ago, grew rather worried when they realized that their route involved a couple of hundred mile transit across a world that had been around for forty billion years in a universe that offered easy access to high-powered psionic abilties.
OK, they were extremely powerful, and they’d be traveling between the two gate-points at supersonic speeds in an armored, armed, and shielded starship with every protection that they could put on it – but they had some idea of just how dangerous the creatures of that world might be, and they’d still need to spend several minutes there locating the next gate.
They pulled it off, but it was a pretty tricky transit. They managed to escape the plants near the first entry point, warded off the first few creatures, but had a hard time with a couple of minor predators that managed to crack the shields and teleport aboard.
Fortunately, once they got out, the micro-organisms and tiny parasites that had jumped to them from the small predators were severely weakened by leaving their home dimension, and could be dealt with magically.