I'd eliminate Orks. In terms of races to add? Hrud. Or one of the other underdeveloped alien races (Dracolith or Draxians, whatever they are).
Orks are brutal if unsophisticated warriors, yes, but if you take out their humor that's ALL they are. No complexity possible, no interesting nooks, like stereotypical Khorne Berserkers. And humor doesn't help much either. It's just... they're not even alien. Merely boring. Tyranids at least have a neat aesthetic, but that's it, again zero depth beyond 'om nom nom'. Villains having some sort of motivation or complexity isn't a bad thing, people. Look at Chaos Marines, or even Daemons. Mind you, taking out Orks massively changes 40K. I'd prefer to have them 'fixed' like the Necrons were.
Post-5th Necrons are awesome. They're parallel to the Eldar, yes, but really quite distinct. Both killed their own gods, yes, but one intentionally in defiance, the other accidentally and caused an apocalypse. Both are ancient, but one slept the aeons and is now awakening and the other evolved through them (through generations) and fights to stop its seemingly inevitable fading. They have distinct aesthetics, which is quite important, and psychic vs. Materium ties. The Necrons are divided but believe they should be united, the Eldar are more or less united in practice even if they're nominally independent. The Necrons enslave other races, the Eldar manipulate them.
The Tau are also worth having, as a xeno faction that is comparable to humanity - all the other major xenos are either elder races or near-mindless (Tau-Humans, Eldar-Necrons, Orks-Tyranids-Daemons... young empires, ancient relics, and destructive scourges.). And the complaints of 'anime aesthetic' make no sense to me. All the 40K races look different, that's part of the point.
As for races to add, again, I want something maximally distinct. The Hrud (just don't make them look like Skaven) as a third elder race, or some previously mentioned xenos species as a third younger race.
All IMHO, of course.
Last edited by VulkansNodosaurus; 07-02-16 at 06:06 PM.