I'm posting this under general discussion since tons of stuff like this has been suggested before, and I think alot of it was rejected. It's really more for discussing about would it be possible and would people want it, rather than "MEVANS PLS ADD MY SUGGESTION IT COOL!!!!!!". I also won't add this to the suggestion forum since I wanted to point out many points (no pun intended) of similiar topic on one thread and in less detail. As a suggestion, I'd have to add more detail to each one and write them out seperately.

How about..

Melee:

Players would be unable to use most melee weapons while swimming. Weapons that would be able to be used during swimming would be restricted to daggers, (shorter?) swords, pickaxes, (tool) axes and (not really a weapon ) hoes. The thing is that using a two-handed weapon while swimming just isn't possible IRL, and I believe it would be fairly simple to code.

Hoes would be added some damage. They are certainly pointy to some point (pun not intended..) and definitly are more dangerous than blunt fists.

Ranged:

Bows would be unshootable while swimming. Same reason as melee weapons.

General:

Cooldown between switching items in hotbar. Now, this might be very obnoxious, and most players would probably hate it, but it would greatly add to the realism and difficulty, and also, encourage players to stick to one weapon type, and practice fighting with it. The cooldown wouldn't be very long, maybe around the time of a 90% attack speed, but it would disable such unrealistic combos such as hitting with a hammer, instantly following up with a spear and then a pike, or the loaded crossbow hotbar tactic. The cooldown would apply to ALL items, making it harder to wall off enemies and adding a requirement to find a somewhat safe place to eat in.

Crouching/lying down. I know a mod that adds this, I've played with it, and it's actually more useful than you may have ever though. One thing I noticed while playing games such as GTA is the importance of taking cover. I used to not care much about it while playing minecraft, and so do not other people, most just think the key to surviving in combat is OP armour and weapons. Something that would allow you to hide behind 1-high blocks would certainly come to use, atleast in my opinion.

Mob-wise:

I know there have been several requests and rejectments for "smarter AI", but is it really that hard? Would it, for example, be possible for an NPC to detect a player's gear, and act according to it? Such as if an unit with a sword detects a unit wielding a pike, it will keep distance and try to attack by charging in, hitting, then backing off agian. Pretty sure I've seen something similiar before.

There was also a mod which had mobs on low health backing off and consuming potions, maybe there could be a slim chance for this to happen with the mod's NPC-s. There would be a chance that NPC-s with low health would sprint away and consume a food item to regen. For hired units, there could be an option to turn it off.

Feel free to share your opinion on these ideas, and share your own.