If they fix the elemental passive, then this guy will be a tanking beast: 5 base dmg shield, plus Photosynthesis ticking for 15 base dmg every turn under Sunlight. And, when he gets near death, you can backrow him with Photosynthesis up and he'll heal a ton. It will take absolutely forever to kill this guy unless your opponent has aquatic damage, or a very high damage combo, or a way to keep Sunlight away.
Darkness has two of those things and is very popular already, so you need a way to deal with that. Possibilities include another Sunlight caster and/or an anti-Crow pet. Soul of the Aspects might be a great option here: has Sunlight, takes less damage from the Crow, has anti-magic damage as a bonus for Spectral Tiger Cub (this also discourages Enchanted Broom using Sweep on the Ancient), and he can choose Surge of Light as a metagame anti-Darkness move, or a big nuke to use under Sunlight.
Conflag teams will also be able to swap weather on you: this just makes it more important to have backup for Sunlight. The Ancient's shield will majorly nerf Conflag's damage output though, so I don't think you need any additional anti-Conflag measures.
Tiny Snowman/Kun-Lai Runt could be annoying. The Giant Bone Spider would help against the Runts, and would also give you a bit of oomph against Anubisath Idol.
Pump is absolutely your biggest fear, but it's not commonly used right now. If Purple Puffer became popular, then you'd need a way to deal with him.
Critters are theoretically a counter for elementals, but the only popular critter is a rabbit, and there's no way a rabbit could ever kill the Ancient through shield+Photosynthesis. Similarly, DoTs can't kill you, and Zandalari Raptors can't kill you, so you don't need specific counters for these.
Crawdad is great under Sunlight, but this might be overkill. Crawdad will thrive under conditions where the Ancient is already winning anyway, and he'll be a liability under the same conditions where Ancient is weak. Ie Crow is a huge problem for both pets, and this puts a lot of pressure on your 3rd slot. You could try Ancient+Crawdad+Emerald Proto-Whelp, but this leaves you with only 1 Sunlight. Ancient+Crawdad+Soul of the Aspects could maybe work in this scenario, but then you lack good options against Deep Freeze, and opposing tank comps would be a pain. Ancient+SotA+Giant Bone Spider might be a good middle ground.