Ground undeads can use a player made travel tunnel.

Since zombies reborn is a long long mod to play (spanning hours). A way to save and exit a game and then return later would be very great for singleplayer users.
Although theoretically possible, it would be difficult to implement- this is because the mod would need to save information like:
day time, day count, all blobs on the map, inventories, players, blob health and likely other stuff.
Ill be honest and say this will likely never be implemented unless there was significant demand, but it will stay here as an idea.

Zombies insta killed if there is a saw on their spawn.

A mostly aesthetic idea from goldenguy. Fog, wind, lightning, snow and rain for example.
The idea could be pursued even further, going as far as adding in seasons to accommodate the types of weather.

This could be an effect of Christmas, We will see once KAG updates.

Add a flight path to merchants on the remote. So that they arrive from the left or right of the event horizon, and not from outer space perpendicular down / up.

Trophies. After the death of monsters, a trophy drops out with a small chance. The trophies may disappear after a while. After picking up a trophy, it can be sold at a good price from a merchant:
Zombie Knight: Old Sword 25 coins
Greg: Heart of Stone 25 coins
Wraith, skeleton: Skull 20 coins

Enable zombies to pick up money.

Add a merchant on a donkey. Comes on foot to the player's castle.

Enable the zombie knight to throw bombs back at the player if they fall next to him.

Enable Greg to pick up keg. It'll be fun.

Add regeneration to undisturbed blocks of earth.

Add vanilla tunnel.

To make it possible to use corpses as a combat projectile for a catapult, similar in effect to a stone boulder.

Add a bonfire. It illuminates well and you can fry meat or fish on it.

Lamp unit, requires a lamp. A very useful block. During the night, nothing is visible and it is necessary to illuminate the surroundings. To do this, I suggest adding a block that is conveniently located somewhere static.

A fire grenade. The return of the second vanilla bomb. Instead of an explosion, everything around is set on fire.

A crawling zombie. If you knock down a weak zombie, it could start crawling. Of course, after receiving a little damage, he immediately died.

Stone rubble. A block from the classic KAG. It is found in ruins or appears after a collapsed tower. Upon contact, it is destroyed.

Statue of Greg. A decorative element that could be used as an addition to the ruins. You can add a revival function, when a wave of zombies begins, the statue wakes up.

Grid block, requires a stone. A block that cannot be passed through, but you can shoot or throw objects through it. It could be used as an addition to the ruins.

Add migrants. Take a tax on their earned money. After death, the player could be reborn in their place. Use them to protect the castle, giving the opportunity to choose the right class and order to patrol the territory. To order to perform simple actions: to dig a stone, a tree, to produce food by growing harvest.

Add various potions with temporary effect. During the action of the potion, the player emits particles, and also next to his health there is an icon of the effect and the duration of the action. Basically, they make modifications by increasing the standard characteristics.
New items:

1. Medicinal ointment 155 coins. Restores 3 hp for some time. If you take damage, the effect of the treatment will be canceled.
2. Health potion 385 coins. The magical properties of this potion will make you fresher! Within 5 minutes, gives + 1 health point.
3. Power potion 325 coins. Your punches will be overflowing with power! For 3 minutes, gives +1 damage.
4. Speed potion 275 coins. The energy of this drink helps to cheer up! For 2 minutes, gives +25% to the attack speed and movement.

Add an analogy to gold - crystals. Thanks to crystals, buy rare items or create new ones.

Add weather effects. Rain, thunderstorm, morning fog, bright night, strong wind. During a thunderstorm, lightning could randomly fall on the map, illuminating the entire map and setting fire to the block. A strong wind could slowly push aside all objects.

If anyone is up to make some maps for the mod that would be cool. Regardless, I will be making some additional maps in the future. Creativity is encouraged!

Some players have more skill issue than others. Perhaps game difficulty could be implemented so weaker players can play easier.
Easy, Normal, Hard, Insane etc.
This is assumed for singleplayer mainly, however server hosters could choose which difficulty to use for their players by accessing the zombie vars config.
Things like damage, tile damage, health, spawn rate of zombies could be edited for each difficulty.

It is not the best

Say that you are missing half a heart, if you ate bread it would heal you completely however the extra heart the bread could have healed you for was destroyed with no actual gain.
For food to be more useful, it would be wise to make it so food is only killed until all of its healing potential is used

Its a bit too difficult to heal as of the moment. It would be wise if stronger zombies (zombie knights, maybe wraith) gave a heart on death as a reward for killing them.

Other zombie mods also have some form of this mechanic, with 'migrants'. There is a good amount of ideas that migrants could be used for in zombies reborn. Ideas like resource collection and defense come to mind. In classic zombies, migrants were used as a way for players to respawn. In base kag, migrants are used to make factories function.

Currently, the way for players to get coins from zombies is by killing the zombies- and then going to their death position to pick up the coins. This causes the majority of coins to get unused as the horde is usually too dangerous to get close to. A solution would be to give a certain percent of the coins immediately to the player whoever killed the zombie (sort of how CTF does coins).

Wraiths are the only enemy that knows where the player is at all times. Despite this, wraiths are stupid and can't move around obstacles to get to their target, so they end up stuck on a one block step or something else.

Zombies that come from below

It is possible to heal more hearts by switching to an archer, then eating food and switching back to whatever class you were before.
If food healed by percentage (e.g 50% of your blob's health) rather than a set amount of hearts it would be better.