I'm going to be completely honest with you and I hope you don't take this the wrong way, but the amount of 'suggestions' that they hear a day.... it's like finding a needle in a haystack but the needle wasn't even there in the first place.
Regardless of my own assumptions, I have written a couple of letters to various department heads, including WDW President George Kalogridis giving input on various things that positively and negatively affected my vacation and I ALWAYS hear back from someone via phone/email about this. Most recently, I had some input on how the Express transportation positively affected my vacation last summer and how I was shocked to find it retired. A nice woman from whichever department relays information to GK's office called me and we had about a twenty minute chat about this, all while she took notes, in which she said she was going to forward to the Sr. Vice President of Operations/Transportation as well as Mr. Kalogridis. So writing a letter certainly gets somewhere, but it may not be exactly who you wanted it to go to.
I know that if you ask to speak to someone (within reason), normally a CM at the concierge can get ahold of someone for you to talk to, whether it be someone from an office, or a Concierge manager of some sort. (As an example, if you request to speak to the GM of Magic Kingdom, he probably has other things to do rather than listening to suggestions all day. One time at AK, a guy was super angry about something and he asked to speak to a manager and I believe it was the assistant to the VP of AK came out to speak with him.), but I'm sure someone at the desk can put you in touch with someone, or take a message with some input. I'm assuming that managers at resort hotels might have some more availability if you want to talk to someone.
The WDW website also provides you with this email form. I've never used it, or really used email for input so I can't speak to how well the system works, but I do know that the WDW resort has a very strong and very active team of people who respond to as much feedback as they can.
https://disneyworld.disney.go.com/help/email/
