While there should be a balance, lately there has been too much retreading of the same Alien species, which leads both to small universe syndrome and unnecessary retcons.
Of the major species:
- TOS brought in Vulcans, Klingons, Romulans, Tholians, Andorrians, Orions and Tellarites
- TNG brought in Ferengi, the Borg, Bajorans, Cardassians, Trills, Androids and Betazoids
- DS9 brought in the Breen, Jem'hadar, Vorta, Changelings
- VOY had a slew of VOY-only species: Ocampa, Kazons (though they're Klingon ersatz, tbh), Undine, Hirogen, Vaaudwaur (underused), Voth (underused), Talaxians (overused everytime Neelix is on screen), Captain Planet Villains (overused in that they were an incredibly dumb idea), space George Constanza, Potatomen, ... though not all of those are major.
- ENT had the Suliban, Xindi and Sphere builders
- DSC mostly retreads Klingons and Vulcans
- PIC mostly retreads Romulans and Androids