I'd love to see somebody pull off realistic Andalites someday. Imagine these telepathic creatures walking around DS9.
I'm sure their morphing technology would mess with Odo's head!But yeah, what people said above. The budget made it hard to come up with a million aliens and have them look like more than a cheap B-movie monster of the week.
Well, they were able to do Centaurs on Hercules and Xena. Granted, they did a lot of close ups and the whole body affect looked a little cheesy, but that was almost 20 years ago...
I'm sure they could do these aliens you mentioned today.