First off you need to make a story thast you will enjoy bringing to a conclusion. While a survey is good to figure some of your approaches, you need to keep in mind you're making the story.
You mentioned BG and IWD, both are great games with a lot of similarities but thay also have major differences.
BG will play more like the OC. The player is the main character and they find companions to help out. The builder designs them, creates their backstory and offers interactions with the player. This is very time consuming but adds a whole bunch of depth to the story. You now have additional tools, the companions, to help tell and mold the story. Torment is a prime example of that. Each companion added to the story brought more content and background to the player.
A IWD type game would rely more on the SOZ party system. Now the player has control over the whole party and their creation and backstories. While this will save time in building you have given up a story telling component. Also with the SOZ conversation, in my opinion, you remove the need for skills. It doesn't take much for a party of min and maxes to be able to power through a conversation and get all the hidden bits, killing replayability. But again this is your story and take what any of us say with a grain of salt.
Do you like romance to be an option or not?
No romances. I find them to be hollow and unrealistic. I always skip over them, intentionally avoiding them since for me I find they break any type of immersion. When they are forced on the player that is just miserable.
Do you like to start with no equipment or to be able to import your pre-made character's inventory?
I would suggest stripping all items and gold and them adding a starting amount of gold and have a equipment store either in a lobby or preferably worked into the starting area. For balancing this will prevent an over powered character from importing in with everything or if a player pulls a character from a low magic/item mod they will not be left defenceless. Also unique items from another mod might rely on tag based scripts for their powers, when that character is imported in the items will no longer work and you will be blamed for breaking them.
What kind of companion characters do you like and what kind do you hate?
If you will be using companions like BG or the OC you need to balance out a party for the player to choose from. That being said, those companions need to have a reason for joining other than because you need X class to round out your classic party. This is the opportunity to make interesting characters to join as well, with a level 15 party you can make some interesting companions with strengths and weakness. This is where I would suggest searching through the vault or nexus for some interesting models. When it comes to companions I always fall back to Torment, the first person(?) to join you is a trash talking floating skull.
Do you like to be able to give witty, unconventional answers?
For conversation start first with your grammar and spelling, and you main core conversation for your "normal" responses. Choose 2 to 3 other type of responses and maintain that same choice type throughout. I don't want to occasionally be jerk, if I'm a jerk everyone should be able to know it.
Do you like companion's bickering with one another?
If you are able to write it and you are using the OC style conversation then go for it. As I said earlier this is a tool you can use. When making the crypt in the middle of a swamp you can add the yellowish fog, but a companion complaining about the choking stench will reinforce the area and add depth to your story telling (if that is what you are aiming for).
Do you like villains who are original characters or canon, famous characters from D&D?
Original villians. Name dropping is fine. So and so gave me this or I studied under this guy/girl, this ties into the FR world and gives the villian a bit more background.
Do you like to meet other canon characters from D&D in a campaign?
Just don't meet epic heros who are known for fixing the crap that the PC is currently struggling to fix. Why would they sit on the side lines and allow a world they saved multiple times fall into misery because the new guy couldn't step up and get it done.
What kind of quests do you like, what kind of quests do you hate?
With a level 15 party you get to do a lot more fun things. Break away from the standard quest and challange the toolset and your scripting. Create dungeons or areas with real puzzles that the player will need to work out. Create or find creatures with immunities and force the player to find ways to defeat them other than just beating the snot out of them. Cursed immortals that can only be hurt by weapons forged on a certain anvil. I fooled around with oozes and swarms, these Critters require a different appropach then most monsters and can bypass or destroy weapons and armor.
Do you prefer a lineal story or a more sandbox-like gameplay?
From experience I would suggest a lineal plot line for simplicity in creating and debugging. Then allow the side quest to be open, allow the party to stumble into a mess of nasty monsters that they will need to run from and return back later for a bit of extra loot and experience. It is also these side quest were you can explor more of the good vs evil or law vs chaos alignments. That way your plot quest line doesn't get bogged down wiht a "good" way or "evil" way to complete it. The side quest will allow the player to define themselves by doing this one and not doing the other one.
Do you like epic, legendary plots (kind of like Throne of Bhaal and Hordes of the Underdark) or do you prefer more realistic adventures?
This one comes to what do you feel like creating. This is your story.
Do you prefer to have your background predetermined (like in Baldur's Gate) or to be able to create it yourself through dialogue options?
I like a bit of a predetermined back ground if it fits and adds to your story. If you are creating an epic plot line then it will llikely add to it. If your a just a hire mercenary on a more mundane story line then it may take away from the story.
Do you like to have a varied assortment of companion characters to choose from or not?
Quality or quantity. Make then good and interesting. The orge mage that joined the party seeking revenge on the antagonist for betraying him. The problem is during the betrayal he was hit in the face with a battle axe and lost half is bottom jaw. Now, using a spell hook script, due to his difficulty in pronouncing spells correctly he will randomly miscast spells with chaotic results. The spell may be benificial, detrimental or prehaps a dozen fluffy white rabbits will appear. I woluld choose that guy anyday over a fighter tank. Just imagine the banter and interjections a companion like that could have...