Here on RPG.SE, we expect all answers to meet the criteria put forth in the Stack Overflow blog post Good Subjective, Bad Subjective. This means answers should "Back It Up!" In our world this means:
- Stuff from the actual rulebook or other relevant sources - with some addition of logic, direct comparisons, etc. allowed. ("How much does this exotic familiar cost?" "It's not listed, but a pseudodragon costs 200 gp, so that seems fair" is fine, that's a direct analogy.)
- Citable experience - have you done it or seen someone else do it? How did that turn out? (This last part is crucial.)
What you are NOT allowed to do is say "how you would" handle that. That's Bad Subjective, aka speculation. RPG.SE isn't for brainstorming or idea generation, and we're not really interested in what someone who's thought about the problem all of 30 seconds tosses off the top of their head of what they'd do. We don't want speculation, we want experience.
In this case, if you have or have used (or have watched an Actual Play video where they do it, or whatever) a crouching rule, that's fine. If you want to answer from the perspective of "there's no crouching rule, but of course crouching might give you more cover according to the existing cover rules for this reason" that's fine.
If you want to say "well how about it takes a bonus action and then you have to rise to kneeling first from prone and it gives you a -1 AC and and..." because that sounds good in your head, that's not fine, and you can expect downvotes and delete votes, because we want to leverage the actual experience of our community for solid answers.
If you say "well, our group implemented a rule where you had to use a bonus action to crouch or stand up, you couldn't move faster than a walk while crouched but it gave you a +1 AC. We ended up never really using it because it was too fiddly | It worked out great for our tactically minded players | It ended up being abused in combination with the DwarfStomper feat | whatever." that's a great Good Subjective experience-based answer.
The difference between that one and the previous is having really tried it and seen in play if it's a good idea or a terrible idea or what mix of those it is.