On the old forums I saw a number of suggestions get eventually used in-game but
and I cannot stress this enough... years could go by before an abstract suggestion rather than concrete suggestion sees development. What do I mean by abstract vs concrete? Well sometime in the first 2 years after I joined the game so sometime in 2010/2011 I suggested 3BC needed a quest with airships like the one out in the wilderness docked by the little pirate village and more thematic pirate loot like spyglasses, compasses, an eyepatch, etc. Most of that eventually made it into the game but not necessarily in 3BC. Although we do have Precious Cargo so maybe? That was mostly a list but rather abstract in that the suggestions were very open to interpretation. Concrete: When they were updating 3BC for epic I specifically suggested they make the great big Arch in the wilderness that leads to the Black Lock an explorer location... and now it is one.
They made mounts inapplicable in taverns from player suggestion.
Cloaks were initially suggested by players. Mounts were initially suggested by players. The concept for Wish For Memories and the item that allows you to pass one BTC item to another character on your account were originally suggested by players.
When we still had Risia Ice Games in the Harbor and Midwinter in Eveningstar another player and I were brainstorming other prizes they could add to the Winter Events. I photoshopped a number of ideas and this was one:
Eventually we got
https://ddowiki.com/page/Item:Void,_the_Endless_Cold , not for the Winter Events, but I'd like to think my suggestion inspired them.
So yes they read/use player suggestions but unless the suggestion is specific to what they are developing at the time its unlikely to be used presently. And if the suggestion is more abstract like a quest seed or quest pack or new feature like mounts then it could be years if ever.
I think its great when players lay out very specific quest ideas, I love to read them but I'm sure I've never seen a dev follow a specific quest script that a player wrote. On the other hand more nebulous, abstract quest ideas seem to sometimes make it in. We were brainstorming a murder mystery type quest at one point on the old forums and sometime after that we got Eye Know Whodunnit, not exactly a murder mystery but still a mystery with multiple outcomes as we were hoping with a search for clues. Now whether or not that's a case of inspiration or just minds thinking alike... I guess we'll never know.