Technically, you can't be hit while invisible - if you use the right tactics and your enemy does not have ESP or Infravision.
We all know that in IVAN you must attempt to walk onto the same tile as an enemy in order to attack it. This also works in reverse with the enemies, and since you are invisible, they simply try to walk in the direction of where the attack came from (unless they have ESP or infravision, in which case invisibility has no bonuses). As such, you can attack once, then move upwards - your enemy will move to your previous position as they are trying to attack what they can't see. You can then launch another attack and move to, say, the left etc.
However this can have problems when the monster moves faster when you do, as they will get one or two hits in before you move.

You won't automatically attack a friendly or neutral character just by walking into them. A confirmation message appears asking if you wish to attack the subject, and pressing any key apart from "Y" will simply cancel the action. The only time guards will attack is when you steal or break something they're responsible for, or attack either them or an ally of theirs.

Also, as discussed elsewhere, putting any item into the coffin in the catacombs will give a random bonus to your alignment with the gods.

A wolf, blink dog, or even just your initial puppy. Feed it ommel bone things. If the object is big enough, you may get the message "X the (puppy/blink dog/wolf) looks extremely ferocious! You shudder."
Note that to feed your canine ommel bone, drop the object on the ground and let them walk over it. DON'T throw it to them (unless it's a ommel bone bone).

Here we see a wild "Unda Mortalis" or "Fountain" basking in its natural habitat. Although it takes the appearance of a man made water feature, it is actually a living organism which is connected to several other "fountains" through a series of "hyphae". In this sense it is more a kind of carnivorous fungus, but is not classed as such.
Normally this species is not dangerous to humans, as they generally tend to walk past, or observe them from a distance. However, there have been incidents where an unsuspecting human has provoked the fountain, leading to it to respond in an aggressive manner.
Since the Unda Mortalis possesses no sense of sight, smell, or hearing, the only way it can sense it is being threatened is by the offending creature to touch the fountain's marbly exterior, or by "tasting" the aggressor should they come into contact with the fountain's "water". Since the fountain's true power lies only in its "water", it only responds to attack upon said water being contacted. Despite this, the fountain can be quite patient and composed in the hopes that the aggressor will simply walk away. However, once its patience has worn down, it will strike quickly and with no prior warning.
This can be in a variety of ways. The most common of which (and perhaps the most confusing) is to suddenly absorb the offending creature and to propel it through the fountain's hyphae, resulting in the creature either being expelled out of another fountain or being thrown out of a wall that the hyphae is protruding out of. Less likely to happen is the fountain absorbing the aggressor and storing it in a sort of gelatinous globule called the "sarlac", where they are brainwashed by small viral bacteria that naturally occur in the area. After this process, the brainwashees are at the full command of the Unda Mortalis, and may be produced from one of the "fountain flowers" to combat any offenders. HOWEVER, sometimes the process of the bacteria brainwashing the creatures can be defective, and as such they end up obeying the commands of the offender instead.
Another, more subtle way the fountain responds to attack is by changing the properties of its "water". Although it is able to induce this process voluntarily, it is unable to fully control the change, and the result the "water" has on that which drinks it can vary from a highly negative effect (such as permanently slowing down an organism's muscular function), to an extremely positive one (permanently accelerating the organism's muscular function).
The last, and possibly most rare occurance, is that of the fountain taking a liking to that which quenches its thirst from its "waters". When this unbelievably unlikely event occurs, a small "spirit" (which is really a manifestation of the will of the Unda Mortalis) will appear to the creature and grant them one wish.
However, one must be careful not to drink too much from a fountain, as it is unlikely to remember you are the same object that it took a liking to, and will resort to its usual defences. Also, it is possible to kill this fine creature by removing all the "water" from its bowl (or "petals" if you'd like), and one must take care not to drink it to death.
Finally, the previous belongings of a past victim of the Unda Mortalis can occasionly be found in its bowl. These are usually rings of all sorts, as the fountain cannot digest them and must regurgitate them to avoid clogging its system.
None know quite how the fountain eats. But it has been said that not all that is absorbed by it sees the outside world again, whether through brainwashed eyes or otherwise...

Now using that and eating ommel bone...

A scatterbrained wand. Does the effect of a random wand when zapped. Has a chance to break itself resulting in a random effect too.

And probably an outlandish idea: The Room of Change Material. A simple non-descript room that looks no different to the others on the level, but anything inside the room occasionally changes material to something random. Upon entering the room there should be some sort of alert, like "This room seems dangerous", "you get the feeling it is not safe here", "Your skin seems to be crawling". The change should affect all bodyparts along with inventory items, but only has a chance to (to prevent everything changing at once) and without an announcement of having done so. The message "Your head suddenly feels like marble!" would definately give it away.
To make things more interesting, there should be a unique monster that spawns in there, and is slow-moving so it has time to get its limbs and such changed. A golem of some sort, perhaps?

Also, some lovely music to provide ambience. Use MIDI for that authentic "old game" feel!

Baldur's gate had a similar "random encounters" system which I wasn't too fond of. You had a chance of being "waylaid" between areas, which consisted of a fight in a generated zone.
IMO as previously suggested you should be able to ">" anywhere in the wilderness, and have a chance of critters appearing (and maybe unique encounters too...)

Night/day actually having an effect would be nice (like making undead and nocturnal creatures appear at night, and general wildlife during the day). Also night/day on the world map. And Attnam if it hasn't got that already (it hasn't changed from snowy day in my experience).

All I have been doing is fooling around with the code, for lack of anything beneficial to do. (See: Tonberry, Santa face)