There are many ways you can go about this; I suggest the best method is to be creative with it.
I made a very difficult obby; THE IMPOSSIBLE OBBY - Roblox and it did very well, as it was something unique, a very difficult obby which saved stages, simple but different.
Regular obbies, as in the colourful ones which are usually seen on front page are a bit over-done and saturated at this point, I don’t see much potential if you made one of those.
There are also ‘story obbies’ which is where you follow a story to get to the end, usually an NPC guiding you through the obby telling you things like “Watch out for the lava!”
There are also ‘tower obbies’ which is a concept similar to Tower of Hell.
Obstacle courses have a lot of potential if done properly, I had several ideas for great unique obbies on my group, sadly… it got locked for no reason and I got terminated, still waiting on someone to resolve that, doubtful though.
Just be creative with your obstacles, try something new, maybe mess with player controls, add crazy things to it, make the player excited to complete your game, I suggest to add savable statistics as it encourages players to come back, but of course this depends on the complexity and length of your obby, no point in adding savable stats if it’s short. Long obbies do much better as if its short, people play through once and then the game will start to die down, except if it makes it on a game-sort on the front page, then it’ll survive longer, but most cases, it’ll just die down.
A theme is a good starting point, find a theme, take it, and find out how you can incorporate various obstacles with it.