I've noticed a lot of the suggestions in the category suggestions thread basically are almost always rejected immediately. There is a clear criteria for a good suggestion, so I'm going to list them here.

1. No Already Existing/Similar Categories Some suggestions are rejected because they already exist here or on another leaderboard, or are too similar. Before you write a suggestion, double check the Official Bedrock Edition speedrun leaderboard, as well as the original category extensions, and even the Java category extensions. Also make sure that your suggestion isn't too similar to a category. An example is Obtain Beacon. That would SOUND like a cool category, but it's basically "Defeat Wither but also get glass and obsidian"

tl;dr, Check if it already exists or is close to already existing

2. No RNG Based Categories If a category is too reliant on RNG to get a good run, then it will likely be rejected. RNG based categories would be grindy/not fun to run. An example is "Obtain Trident" which existed in the early days of MCBEUCE. It's rare to get a drowned with a trident and just as rare for them to drop it.

3. No Long Categories Most people who speedrun here are most likely only going to want to spend 15-20 minutes maximum speedrunning a category. If your goal in the category exceeds over 30 minutes long, it will likely be rejected. Not only will people not run it, it will be a hassle for the mods to verify

4. No Chest Categories A chest category is defined as an Obtain Item speedrun, and that the item that is needed to complete the speedrun can be found in a structure chest. There are already quite a few categories like these here and there likely won't be any more added

5. No Unverifiable Categories If a category has an objective that has an unclear objective, it will probably be rejected. It will be hard for the mod to verify runs if they don't know whether the speedrun is over or if the objective was unclear in the first place

I'm getting lazy I'm doing shorter descriptions

6. No Overly Difficult Categories Don't make your category so hard to the point no one wants to run it.

7. No Stupid Categories Things like Dirt% are a waste of time

8. Check If A Suggestion Has Already Been Rejected Self-explanatory

Overall, just be creative! If you suggest a category that is fun to run, not too long, not too hard, and original, you would have a fair shot at least being considered.