Do people tend to lose interest if you offer to allow them to help in coming up with parts of the world building?
Not at all! If anything, my strategy for building interest early on is to post an interest check thread while the RP idea isn't yet fully fleshed-out, and then I end the first post with something like, "so, I'm still not sure where to go about X/Y/Z things, any suggestions on that?"
Encouraging discussion and participating in your interest check thread has quite a few benefits. It allows players to get invested in the world early on, because you're starting them off by having them think about what they want to see. And if a player suggests an idea, and you then go forward with that idea, the player is now extra-attached to it and is more likely to stick around to see that idea come to fruition in the full RP.
It also helps you get an idea of what people do or don't want to see. Ultimately the RP is your idea, and you have to decide where you want to draw the line when it comes to creating your own world vs pleasing your audience, but if you're specifically aiming to make something that people will like? This is a good way to steer your RP in that direction. Because then you already have people telling you exactly what they want to see. (Of course, sometimes you'll have a group of potential players with wildly different ideas that all conflict with each other. But if there's one idea that's really popular with a group majority? Then it's definitely worth considering.)
Also, all that extra discussion means your thread will keep getting bumped to the top of the forum list, which means more people are likely to see it. This is why, even if I don't have any specific questions in mind about what people want to see, I'll still always end my interest checks on
some sort of invitation to participate, even if it's something as simple as "let me know if you have any questions or suggestions for what to add."
Another pointer I would add: make sure the
main premise of your roleplay, important stuff like the setting, the sorts of characters that people can play as, and the main plot, can be summarized in a paragraph or two. And then make sure those 1-2 paragraphs show up
early on in the interest check. Ideally it should be the first thing your players read. That way, you can get people immediately hooked on your idea. As opposed to, say, starting people off with five meaty paragraphs of intense lore, making impatient people like me try to skim through all that lore and think to themselves "ok yeah this is nice and all but what is your roleplay actually
about", and then give up when we can't get an easy answer to that question.
If I had been given an interesting premise up front, I would probably have been more inclined to read more of that lore and actually get invested in it. But if I'm just trying to quickly decide whether or not an RP is worth investigating further... yeah, having loads and loads of info to sort through is just annoying. I'm not saying that huge amounts of text are guaranteed to scare people away, but it should probably be more of an optional read in these early stages.
On a similar note: formatting is all fine and good, but don't let that make it harder for players to quickly get an idea of what the RP is about. I know that when I'm browsing through interest checks, I'll often just hover my cursor over the thread title, so I can read the first line or two of the thread before I even click on it. If that first line or two sounds promising, then I'm much more likely to click on the thread and read further. But if that first line is really meander-y and tells me nothing about the premise (as in, something like "hey, so, here's an idea for an RP that I've been thinking about doing. Let me know what you think about..." etc), or if that first line just consists of nothing but code, I might not bother to click on the thread at all.
At the same time, though, it's a good idea to put
some effort into making your RP look visually pleasing. If a webpage is badly designed, people will be less inclined to want to read anything on it. The same applies to threads. On a similar note, it's also a good idea to chunk up the information so that it's easier for people to digest. If you're going to include a lot of lore, split that lore up into sections, and tuck each section neatly away in an appropriately-labeled spoiler tab. The most important information -- the stuff that players will specifically be looking for when they're deciding whether or not to join, like the main plot -- should be placed somewhere that players can easily find it. If it's not at the very top of the page, then at least make sure people can easily skim to find it.
Proofreading, of course, is also a good idea. If someone clicks on your interest check and the first thing that they see is a typo or a glaring grammatical error? They're more likely to make snap judgements about your competence as a GM based on that.
Lastly, the biggest piece of advice I think I could give is: pay attention to what makes
you want to join an RP. And I'm not talking about which genres or tropes you most enjoy seeing. I'm talking about,
presentation-wise, what makes you want to click on an RP? What makes you want to keep reading? And what are your pet-peeves? Your turn-offs? What makes you decide that a thread is or isn't worth your time? As you may have noticed, a lot of the advice I gave was based off of what I personally find to be (in)effective at making me want to read an interest check thread. Obviously not everyone's going to have the same turn-offs as me, or as you, but, looking at your own turn-offs is a good place to start when it comes to putting yourself in the shoes of your potential RPers.
Look at your RP from the perspective of a potentially-interested player. Would the intro/hook be enough to pull you in? Would it be easy to find the information that you need, given the thread's formatting? Would anything about your formatting/organization annoy you as a potential player? Again, don't focus too much on the
ideas being presented, but focus on
how those ideas are presented. Because, often times, the problem isn't that people don't like a certain idea. The problem is that the people who might like that idea, aren't reading your thread.