As I'm sure some of you know, there currently appears to be a bug with commercial demand and parks, where the presence of parks causes a massive plunge in commercial demand. If you go in and delete the majority of your parks, you'll see a large increase in commercial demand. This has been noted on both the Steam forums and Paradox Plaza.
I think I've found a workaround until this is fixed, courtesy of some detective work by a user on Paradox Plaza, who supposedly looked at game files and discovered how demand is supposed to work (at least somewhat). If you're interested, here's what he had to say:
The second part of the formula is where parks and special buildings kill off all of your demand.
It is based on a ratio of the number of visitor spaces available (includes capacity of commerce selling goods, people visiting parks, and special buildings) to the number of visitor spaces unused. The ratio is 3:1 in this case, so if 75% of your visitor space is taken up, it = 0 added demand. At 6:1 ratio, it hits +50 (85.7%). At 2:1 ratio it hits -50 (66.66%).
I think tourists are supposed to help offset the impact of the parks and special buildings, but I can't seem to get anymore to come into my city so they don't help me out much.
The Fix: create your own parks using the asset editor. Under park properties, there will be a Tourists option. Click that and change the numbers all to 0. I am not a programmer and I have no idea what this does, but I'm under the assumption that it changes the number of visitor spaces to 0.
After doing this, I went into a saved game. Deleted all my parks and watched as commercial demand rose. I then added my custom park and saw no change in commercial demand. There were tiny ripple effects each time I added a park, but that was true across both residential and industry too. I added a total of 15 parks. I then added a large park with trees -- not custom made -- and watched as commercial demand dipped considerably. After adding a large plaza and a small playground, demand plunged into the abyss.
So for now, it looks like a viable workaround until CO gets this sorted out.
EDIT: I should add that you MUST demolish your existing parks prior to or after adding your custom parks for this to work.