[2020] Am I wrong going Night harveser into Horizon focus on Annie ?

Your build sounds decent but I would encourage you to think when other Mythics could be better. Most of the items that people are building on Live are familiar but not necessarily optimal. I'd expect them to get closer once the season starts and people have time to try out different things. Theoretically, I'd expect Rocketbelt and Night Harvester to be situationally good. (harvester if you are taking lots of group fights and Rocketbelt if you are playing against a champion that is hard to gap close against eg. Orianna, Xerath, etc.) I would not expect harvester to be performant against someone like Xerath at diamond or above as you are paying a lot of opportunity cost by not being able to force engages with Protobelt. However, if you're not that high, then kitey champions are probably frequently mispositioning, so Night Harvester might still pay off. At high ELO, I wouldn't expect that to happen as often, though. Liandrys is likely decent against multiple tanks.

I've only played about 5 Leblanc games this Season, but Lich, Nashors, Void, Banshee/Zhonya and Cosmic Drive have all seemed situationally viable on her. Nashor high AP value lets you one shot backline minions and perform slightly better in the side lane, especially against more tanky oriented fighters that you can kite and auto safely. I would go Nashors if I knew I was going to be getting value out of playing the sidelane, especially if I was going for more macro oriented with TP.

For Mythic, I think Ludens and Night Harvester are both decent, though you have to be a bit more careful with mana on Night Harvester and going Doran after Corrupting is mandatory, rather than Ludens where you can go straight into Mythic.

Out of these, I think Cosmic Drive is the most underrated. In a fast paced game, where fights are constantly breaking out (most solo queue games), I would expect Cosmic Drive to be quite performant, as you can capitalize on your low W cooldown for fight uptime and damage.

Ahri, Annie: Situational if you are snowballing and going for burst build.

Lux, Neeko, Lissandra, Syndra, Ori: Mages that can trigger either the Immobilize condition or the range condition easily.

Velkoz, Xerath, Ziggs, Zoe: Poke champs that can trigger the ranged condition easily.

Untested:

Seraphine: I think it's good on her due to E and R, but it might not be better than Mythic -> Rylais/Cosmic Drive -> Rylais/Cosmic Drive since she is very good with Ability Haste and the Rylais for E synergy.

Taliyah: I think this item is pretty underrated on Taliyah. It is quite easy for her to trigger the max range condition. TF: If you are snowballing and don't want to go Zhonyas or RFC 3rd after Lich Bane, Horizon could be a good option.