What will make you invest further in the game?

Below are what I would spend coins on:

1. More Pokémon storage space. Hope they up the max limit before Gen 3 is released 5-6 months down the road.
2. Infinite Incubator
3. Pokémon customization. I don't mean create your Pokémon. I want to dress up my Pokemons with suits and hats, etc.

Max Potions or "access to pokemon center", Bike, & 2nd Infinite Incubator as suggested earlier

The removal of spoofers from the game for sure. Why investing money when others go to central park or tokyo for free?

Discounted price on multiples of 3 or 5 incubators, the way they have for lucky eggs.

Evolution items for sale…
(STILL only the one Dragon Scale I got on the first 7 day streak! : / )

Sell max or hyper potions and revives.

Increase pokemon storage cap, and improve sorting by giving us several classifications that work like "favorite" that we can filter by, thus making the larger storage more useful.

Improve the endgame by adding PVP battles, fighting cheaters, etc.

Add a way to show off our outfits (and decorated pokemon) on some official website if we want to. Maybe include some stats on our pvp/gym performance.

1. Breeding, knowing it advance what specie you will get from an egg is another way I would spend money again and it should be implemented soon, I am tired of tentacool, paras and goldeen hatches even when gen2 has been released of the last 40 eggs combined (2k and 5km) I have only hatched a Shuckle and a Phanpy, that is 2/40 of "new" mon, this way I will never pay incubators for sure.
2. Infinite incubator
3. Bulks of 5x and 10x incubators with a good % discount
4. More mon & storage space, I need more bag storage than mon because new berries take out space from other items I want to have and I don't like to throw away anything than pokéballs, regular potions and nanab berries. It really hurts when I have to get rid of rasp berries or other items to keep spinning stops.

I would like to purchase a sightings that I can toggle between "nearby" and "pokestops." I hate not knowing 6 of 9 pokemon within 2 blocks of my house because of 2 nearby pokestops.

Maybe i would invest if there was a possibility to have more than one pkm walking with me.

I would surely invest more in incubators, if their order on the selection screen was changed.
It is an imposition to be forced to scroll throught blue incubators until i find the red one at the very end. Now, i surely will never have more than three blue incubators at any given time.

I will throw money at their face for the ability to reroll Pokemon moves.