Forwards' books are definitely worth reading, and highly non-traditional aliens. The cheela live on a neutron star at a vastly accelerated timescale (and are very tiny). The flouen are surfing mathematician amoeboids.
These types of Aliens sound very interesting to me. I may read these books. I like the idea of aliens being basically unrecognizable because people do not immediately think to investigate truly non-terrestrial environments for the possibilities of organic and inorganic life forms.
Also I tend to like the aliens proposed by Larry Niven in his works.
I don't know if my experience will help you or not, but to your more general point one could take the common approach of the race/mass type/species mode of alien creation. But that is far from the only possible approach.
I have two sci-fi games I wrote and play, or GM more accurately. In one there exist mass races/species/types, but there also exist entirely unique (that is singular, they are one of a kind) life forms. An intelligent ship, AI that is not limited to physical bodies or not limited to robotic bodies, intelligent artifacts, biologically engineered creatures and organisms, animals that possess human level or greater intelligence but do not act or behave like people at all, but instead behave like animals.
In the other game every player is an alien, though aliens may be of several different types rather than particular species. For instance there are intelligent machines, but which are not typical robots, such as BOLO-like characters. Then again there are scientific and technological Chimeras. These are like mythological chimeras but produced by genetic experimentation or other technological means, rather than by magic or supernatural force. A man for instance might appear like a man but have the olfactory senses of a polar bear, and the visual capabilities of a hawk and a goldfish (which can see in ways and in spectrums humans are unable to normally perceive). Or a man might have the proportional strength of a gorilla. Or aliens might combine Human, animal, and even alien genomes to produce entirely unique chimeras of very odd and astounding capabilities. There are also multi-dimensional beings in that game, etc.
So you can think of the typical route of creating mass races/species of creatures(and this is unfortunately the same problem all too evident in fantasy games as well, where far too many monsters and creatures are not unique, and instead are mass-produced, limiting their true effectiveness and monstrosity). But you can also think of creating entirely unique and individualized aliens, machines, and even Chimeras, creating artificial and/or unusual combinations which produce never before known aliens.
Also, when it comes to alien construction, I tend to try and think like both a bio-engineer and an anthro or xeno-pologist. What is exactly that makes one alien? Is it only physical characteristics, is it behavior, is it spiritual belief, psychological outlook, mental capabilities, social custom, etc? All of these things, or certain combinations? And what does that mean? Are aliens monstrous, horrible, horrifying, just different, misunderstood, gracious, clever, cunning, unique, atypical, inquisitive, fanatical, religious, cynical, perverted, composed of cells, made of sulfur, unable to speak, beautiful, helpful, violent, helpless, or compatible with man? In short what does it mean to be truly alien, just as in fantasy games I think it behooves one to ask what does it truly mean to be monstrous?
For instance in many ways my Great Dane and Saint Bernard dogs are very similar to me. In some ways they are definitely very different from me, but I in no way consider them "alien." But suppose I had grown up on a planet where there were no dogs and only later discovered them. Would I be different form never having known dogs, would dogs be different without the influence of man? How long in such a case would it take for men and dogs to become allies and non-alien to each other? In such a case would we remain alien to each other for a long time, or quickly somehow adjust to what is common in our natures? If we grew up on entirely different planets how much would be common to our separate natures?
Anywho I'd just say in creating aliens, what is alien, and why is that so? And I wouldn't necessarily collapse that observation to a mere accounting of physical characteristics. But I'd try to think of the creature as a whole, and of what is possible on both the level of species, and on the level of the unique individual.
Good luck to ya.
