As for Nico, she's just another anime chick that's too anime-looking for DoA and has some random ass moves that will probably be called NiCOFu or some other bs...
Kinda late on that one, since NiCO is pencak silat, and absurdly enough, she actually has pencak silat moves on her toolkit, just with lightinings and teleports.
Anyways, my opinion on both characters.
Diego is cool, his moveset is the closest we can get to Kiryu without actually getting Kiryu. American character with roots from latin america countries, cool design, i like his scars, shirt is generic, but i didn't saw anyone complain about the "bonita" shirt that laura has on SFV (that is way more generic and makes way less sense).
NiCO, Nico has some clever stuff about her design as pointed out by other members in her thread. Her moveset also looks fun. Blue hair, i'm not against it, we already have natural purple since doa 1, then we got natural white, and natural pink. DOA was always anime, and it will still be. But her design doesn't appeal to me much, i don't find she particularly awesome. To the point that she is just a reference to some anime just like Honoka is... Well... that is probably 100% true, but Kasumi was also just Mai Shiranui with different colors when she was first made. So i'm holding any real opinions on both as characters before i see how they are developed as i'm still holding out on that side for marie, Nyotengu and Honoka for years now.
But i think both are solid designs, both fit in what DOA stablished as rules for its world and what is possible inside of it, without stretching or anything like that.