Mistakes I've Made
*My game is very large. However, as a storyteller... I find it difficult to tell "short" stories. But, that's okay, because my large game has given me a lot of enjoyment in designing it. The forums here tend to keep my motivation at least existent, and making slight bits of progress just keep me working on my game.
*Initially had a feature of my game be a "morality" system. After obtaining feedback here as well as analyzing the way me and my friends approach "morality" systems in games, I realized it was a waste of time as well as very silly. I opted instead to have choices that were more thematically appropriate to the story being told.
*Initially had my "Sins" in the game (a feature in which players equip items that nerf the crap out of their stats) require a "Time Limit" run out before the items transformed into something insanely beneficial. This was a feature that added endless frustration in player tests as well as didn't serve the initial design goal of implementation. That goal was to give the players a means of overcoming the character's biggest personal flaws, which could and would affect the story as well as interactions within the party. Nerfing stats simply provided too much punishment and not enough reward for what was intended to be accomplished. Now, the items still "nerf" stats, however, they open up new dialogue options, new choices, and even new questlines while they are equipped. These equips are also signposted much better and have appropriate flavor text (as well as some of them having effects that aren't just nerfing a stat) so that players know there is a reason to equip these items. Finally, even if these items are "transformed", the dialogue choices remain for the rest of the game so players can safely unequip even the new equipment once it's purpose is served.
*Too hard of a start. I wanted to provide the feeling to players that they needed to scrimp and scrounge and save for every GP and every consumable and every bit of progress they made was earned. The problem was that the player starts with no equipment, no money, no consumables. This made it very unapproachable for even "hardcore" gamers looking for a challenge. The start had to be "reworked" in order to provide less of a "Dark Souls" feel, but still a challenging feel. Player starting position was moved, signposting on the first steps of the journey were improved, players are given a free weapon and 10 free healing items to start with, and the monsters in the immediate area were given 100% drops on a Quest Item which was then also able to be sold for small amounts of cash to buy more consumables. There was also a shop put close by to buy consumables and sell the Quest Items (if the player needs to), and a Questline was created in order to provide the player with their first set of gear and some stat ups by just walking around in circles near where they started. This starting zone was made more "player friendly".
*Bad Intro. I initially had an intro that is like most of the standard RPG Maker intros you've ever seen. Vagueness. Mystery. Lots of people and terms and stuff being thrown around that you don't know anything about. The intro was very "off putting" to play testers and they'd often ask me a ton of questions about the setting and what things were without wanting to wait to see if/when I'd reveal that information. I opted, instead, to have an intro that actually set up the plot and didn't use anything "unfamiliar" to a player except the main Antagonist of the setting. Each part of the intro sets up the events that lead to the current events of the game and sets up enough of the world that my players stopped asking me questions right away. However, this intro is still pretty long, about 10-15 minutes depending on reading speed, but at least it gets across the crucial information it needs to.
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That's... my short list, I guess? I'm sure I'll run into other issues as I get far closer to demo release and begin having strangers playtesting my game and being more rigorous. But, those are the major issues I've had from "concept" to "actual gameplay exists".