However, what races would you want to see in future series? Would you want them to just use all the tried and tested (Vulcans, Klingons, etc), examine aliens that only had a single episode where they featured (eg Deltans, Ullians), dig up ones we only saw in passing (eg Betelgeusian, Saurian), or would you want all new alien races introduced rigth across the board?
I'm a quality over quantity guy. Either establish that they are long lost cousins of humans, or have fewer and better developed aliens.
For me, less is more. Personally, I think the galaxy is a little too crowded. I think the more that races have a connection and/or shared ancestry, the better. I also would like to see a focus on TOS aliens, like the Andorans for example, as Enterprise started to do. In fact, that was part of Enterprises failing...I had no use for the Xindi, but would have loved to see more of the founding members of the federation. When you think about it, once Enterprise started doing just that, it started getting good.
I've never been too bothered with human-like aliens. I never really found the argument against the "forehead of the week" all that strong, and personally, near-human aliens are something I PREFER. I know TV shows have their budgets to consider, and as long as the design is interesting, I am okay with it, because it is more about the story than anything else.
TOS aliens often were virtually indistinguishable from humans, so the fact that TNG-ENT attempted to introduce aliens with cosmetic features to give them an alien appearance, should be lauded, not criticized.
I noticed when complaining about the "foreheads of the week" that Klingon foreheads were not complained about.

However like a head plated dinosaurs they seem to use theirs as a form of natural armor which given the rest of their culture is believable. I think the key to prosthetic faces of aliens is that they have a purpose the viewer can easily identify. Otherwise it's just wasted makeup.
I think this is a good observation. The make up needs to look like it serves an actual purpose other than to simply look non-human, but I also think that if a race is explained as being distantly related to say human, Vulcan, Klingon, or Andoran, you can get away with slight changes.
For example, if Bajorans or Betazoids had been explained to have some connection to humans (IE preservers), the slight changes could have been explained by genetic drift. Another example could have been that the Kazon could have been distantly related to Klingons.
I know that may be a stretch for some, but if you think about it, we already see that with Vulcans and Romulans: they have the same ancestry, but now have evolved to be different just enough to be distinct species.
I would like to see more aliens. However, I'm also of the opinion that on Starships similar alien types would serve together. For some alien species 72 degrees F would be far too hot or far too cold. These aliens would probably not be serving together on the same ship. You would have aliens used to a certain temperature or certain gravity all serving together. This way to ships environment could be universal throughout. I think this would limit the number of types of aliens we see on any human inhabited ship. For more diversity could be found on non-human Starfleet ships.
I agree with this. The fact that species who serve on the same ship in the same environment, are going to have similar physiologies. So, I think that is sufficient enough explanation for human-like appearances is good enough for me. That is for aliens who roam within humanoid environments.
For aliens who aren't able to function in humanoid environments, environmental suits are a good solution. The Breen come to mind, and in fact, I think it was a quite effective use of suits, because it left what they actually looked like to the imagination, yet they didn't have to spend a fortune on make up or CGI.
This leads to something interesting: in Star Trek TOS, androids look almost too human.
However, Data seemed to break that mold, and as a result, he did encounter some prejudice. Unfortunately, holographic characters have sort of made android redundant and obsolete, but I think it would have been interesting (especially if Data had encountered one) if we had other android characters who didn't look quite human: IE metallic appearance (Voyager did this one time), or maybe androids with a skin that looked too synthetic, etc.