Actually, I see that each generation is getting more creative than less, for gen 1 pokemon were simply based on real animals, while gen 2 did the same, but also anthropomorphised diffrent concepts, of course,
I see some Pokemon from gen 1 and 2 that I find uncreative, Voltorb is an example, then in gen 3 and 4, the creators then mixed Pokemon evolution with Darwins evolutionary theroies, such as Trapinch looking nothing like it's evolutions Vibrava and Flygon, for dragonflies actually evolved from ants over the millions of years the Earth has had life on it's surface, of course Eevee and it's evolutions (excluding Leafeon and Glaceon), despite being made in earlier generations, actually focus on how animals adapt to new unfamiliar enviroments and become new, but related, species, just look at the many species of lemurs there are on the island of Madagascar.
And for generation 4 having "run out of real animals to Pokemonify" I dissagree, as there are still many animals that have yet to become a Pokemon, dolphins and kangaroos have no Pokemon counterparts for example (seriously, Kangaskahn looks more reptilian that marsupial, it basically looks barely anything like a kangaroo), but at least since many species of animals have variety, like cats, dogs, and fish, there areover a million pssibilities for each Pokemon, the creators even mixed animals and plants with inanimate objects, for example, Buizel and Floatzel are a mix betwen an otter and an inner tube/river raft.
In conclusion, I say that Game Freak is actually making more complex Pokemon in design and ability, the first and second generation Pokemon were simple in appearence, but in generation 3 and 4, the Pokemon designs got more complex and detailed, so I basically see that Pokemon is getting more creative than uncreative