Abilities have been present in Pokémon since the 3rd generation, and continue to introduce tons of gameplay elements that go beyond the basic stats and attacks. From simply powering up moves of a specific type to completely altering forms of Pokémon, abilities have near infinite potential to further vary our Pokémon.
As it stands now, the gym game is very simple. Stemming from the original Pokémon games, some of the simplest abilities come from each and every starter - and are exclusive to them. Overgrow, Blaze, and Torrent increase the power of grass, fire, and water moves respectively by 50% when the Pokémon is below a third of its health. Suddenly, that Venusaur you've been shredding multiple Vaporeons with got a burst of strength to power through the gym's last defending Pokémon! However, due to GO's simplified nature, the boost given by these abilities would likely have to be toned down to an increase of 10-20%, similar to how super effective moves only deal 25% of extra damage in GO (as opposed to double like in the main series games).
The popular original Eeveelutions - being Vaporeon, Flareon, and Jolteon - also have thematic abilities from the main series games that can be implemented into GO. They possess abilities called Water Absorb, Volt Absorb, and Flash Fire respectively, making them immune to their own typing! Full immunity isn't present in GO, so these abilities could simply further reduce the damage these Pokémon take from their own type.
The amount of small damage-altering abilities are fruitful. To list off a few more examples, Nidoqueen and Nidoking possess an ability called Rivalry, making them deal more damage to Pokémon of the same gender, but less to Pokémon of opposite genders. Snorlax has Thick Fat, reducing the damage it receives from fire and ice-type attacks. Beedrill, Scizor, and several other bug-types have Swarm, which boosts the power of their bug-type attacks when they're below a third of their HP (seems like it would be crazy good with Scizor's Fury Cutter!). Golem, Skarmory, Steelix, Magneton, Forretress, Shuckle, and Donphan have Sturdy, making them immune to being knocked out in one hit (obviously this wouldn't apply to GO, but here's my thinking: perhaps this ability could prevent them from being knocked out with a charge attack?).
Moreover, Pokémon can have one of either two or three different abilities, further differentiating them from others of their species. And while a couple of light damage-altering passives would be unlikely to shake up the meta as much as we'd like, it's important to start somewhere. But there is so so so much more to abilities than what I've presented here, and that potential will only evolve with the game. To make this a relatively short start, I'll be writing about abilities that could interact with future changes that we can expect to see sometime in GO's future, such as weather and status effects, tomorrow night (likely going to be an edited add-on to this post, but there'll be soooo much more to theorize!).