"It seems all the other creatures are heading for the surface. Go to the surface or rest?"

This shows the two modes that exist during a certain time. You can either go into a fierce survival mode which may reap many rewards, or heal a portion of HP(10) and explore some more.
This mechanic will turn the tide of the battlefield by weakening the strong, and strengthening the weak. But only superficially. Because although the strong are losing HP fighting/surviving missiles on the surface, they may also gain nutrition to level.
But if your HP is low, there is little chance you will gain any benefits from going to the surface.

Failing on the surface can put you back, because you will be too weak to venture out of your nest.
What are your options? Perhaps units should regenerate life very very slowly. 10 HP every exploration round. Unless you find something to heal you, you will be too weak to for the next round, but perhaps you will have enough life when the next next round comes.
So failing on the surface puts you out of "Surface Adventures" for 2 minutes.

Players that do succeed will probably also have lost a lot of HP, but have gained 2 or 3 Nourishment. Compared to the average 1 nourishment when exploring.
Another benefit of going to the surface is finding out who is going to be injured for the next round. You can tell your ally to take out the weak king.

Stealing is another mechanic, though it is risky. A player's nest will slow down an enemy heavily,
and if a player returns to their base, while a burglary is in progress...the will be able to deal a good ammount of damage to the theif as he/she sluggishly tries to escape from the nest.
Range will also come in handy when stealing. Less running is required to exit. Damage will also help as will speed and all other stats. Stealing should be more beneficial than exploring for Nourishment, as well as a good way to slow an opponents progress in leveling.
Stealing a nourishment should take long enough that the player could be in danger. The player who's base is empy could either be on the surface, or in the field...so the time it takes to steal should be a equaled to the average time it could take for the player to return.
I'm thinking an average of 20-30 seconds might be perfect. And if the player is skilled in damage, perhaps 12. The player's ally could also come to stop the burglary. But the damage is done, and the theft can continue from where it started next time if the nourishment has not been used.

Switching between UI and no UI:
No UI means that the player does not know who is stealing from him, and there is a chance here or she will get lost. UI is necessary for upgrading. But chatting is still possible.
Perhaps UI only exists at the bases. So far there no reason not to implement this.
Players should start at their bases with UI, and when they leave their base, they lose UI.
Players will need to develop a keen sense of direction and memory in order to navigate the world.
There is only 1 type of egg for raising your pet. But how you raise it will determine what it does.
Pets have to ability to get as strong as players.

Pets: How will they level? Perhaps they gain experience on the surface. The longer they last, the more exp they gain. Per second. They start very weak, but can regenerate life faster.
For example, just 5 seconds will level a pet when it is extremely weak and level 1. Chances are a level 1 will die in about 10 seconds...due to slow speed and little HP. So each second on the surface matters. This is also very fun seeing a weak and worthless unit, grow to the ranks of the strongest. Their growth rate is fast...as we have decided...