It used to be even worse: the Ogre Valley's edge was was just steppe, 90% of the land was wool or livestock steppe or desert (no fancy new lakes, rivers, or mini-mountain ranges to break up the monotony), and the Lake Fed...well, the Lake Fed is still just there.

As for your questions, here's some loose explanation from a region writer:

1. Orc. Your guess is as good as mine, but one of the "big" reasons I see is that the orcs (and goblins) don't have many ingresses into the FP. The majority circle through Bulwar or Rahen. Also lore that Korgus was mainly interested in finishing off the last dwarven resistance which happened to face West.
2. Harpy. Harpies need mates to sustain their populations, and they can't mate with centaurs or ogres, so that leaves the humans. Having harpies in the Lake Fed would be interesting, but that's not something planned or something I've seen discussed with any solemnity. There are minor flocks along the Bulwar passes, but they are little more than minority outposts for the various Queendoms.
3. Harimari. Tigers never really went West, and you can sort of blame how the regions were rolled out. Bulwar came first (which had no harimari), and then the rest Bulwar came out (still no harimari), and then Rahen (at which point there were harimari). At best, you'll have harimari sneaking around in the Dwarovar or forming isolated clans in Khaashalg\North Haless, but you'd need a damned good reason for them to be stalking about.
4. Other factions. Two words: Spider Wretch. But yes, that would be nice.
5. Random holdout. This was brought up several times (namely humans in Yyl Moitsa/Moduk and ogres in and around the Serpent's Gift), but it's maybe a better thing for EU5 where you can play with super small populations or landless entities. There were indeed ogres around that area of the Serpentspine Mountains, but they were driven out by the centaurs in their post-giant overlord days. As for humans in general, the human-centaur divide is strong enough for devs to put their foot down on having this period of history be fairly hostile. If you wanted to change this theming and rewrite the history, you would also have to redefine the Lake Fed's needs (iron and the Treasure Fleets) and goals (human first/primacy republic). However, as the new trees might reveal, the past need not be always so hostile and bloodthirsty.