ToME WikiGhostBuster Presently relationship with monsters are quite limited. Mostly you can attack them. There are talking and giving commands but they are hardly used a couple of times in the game. You can tame but it can only be done with magic. There are monster relationship related skills (monster-lore), but it only determines your number of companions (I think). And concerning pets/companions either you can have some or aggravate and have none. I suggest some improvements over these possibilities.
Talking/giving
Talking to a monster ('Y') is only used in a couple of quests presently. Why couldn't it be a way to try to tame a sleeping monster (with a small chance). If you succeed the monster becomes your pet. Otherwise he attacks you (and maybe wakes up the surrounding monsters). The taming chance could be improved by monster-lore and charisma (that would not be only useful to get better prices in shops this way). Some equipments could also helps in taming (see masks below).
You could also try to tame him with gifts ('y'). You could try to tame a person/troll/... with a good (expensive) item, an animal with great food (athelas, meat from a rare monster,...) etc. In either cases, the gift will be lost.
It should also be possible to offer gifts to friends (pets/companions). This would improve their abilities and insure a better fidelity. A ring of speed could slightly improve the speed of the friend, an armor element its protection, etc. The number of gifts should obviously be limited (maybe monster-lore dependent?) and the gifts will always be lost (it is NOT a way to have extra equipment slots that you can get back by killing the pet/companion!). You could also give temporary gifts immediately used (in an unlimited way). Obvious ideas are potions of healing, potions of resistance, potions of speed, food (monster could eat and be starving), etc... I think there was such a possibility in adom...
Aggravating
Presently, either you aggravate, either you can have pets. One could imagine a finer aggravating system. For instance, you could aggravate a specific monster class (orcs, dragons, etc). A *KILL dragon* weapon would always aggravate dragons (except for some powerful artefacts), but still allows to have other pets. And artefacts could also be aggravating for a specific race (orcrist MUST aggravate orcs!!!). You could also have unaggravating items. If you have an (all) aggravating weapon, but wear something that unggravates dragons, you can still have dragon pets. This make aggravating weapons less annoying. This could also lead to interesting pets strategies (is this smart to have a companion mystic if you aggravate spiders 
). And character race could also be aggravating (shouldn't an elf always aggravate dwarfs for instance?).
Aggravating could also not only forbid you to have pets of one or several races, but if could also reduce your stealth for monsters of these races. And, why not, have some positive aspects: for instance small chance of scaring the monsters of the aggravated race.
Masks
I suggest a new item type that would be a mask (kind of a disguise) with its own equipement slot. A mask of orc would make you look like an orc (but still have your original race abilities unlike mimicry). Typically, none or few protection, but it would boost your stealth with regard to orcs (maybe +20 or more), increase your orc taming chance, and unggravate orcs. But it could also aggravate others races (dwarfs for instance!), and maybe add some penalty to searching, infravision, dex, to-hit.... Powerful masks (for instance masks of demon or of dragon can be quite useful!) would aggravate all other races leading to specialized characters. Masks would be relatively rare and expensive items, and you could have mask shops, ego masks, artefact masks. Immediate ideas are a charming mask (to improve taming by talking or giving), an horrible mask (small chance of scaring the monsters), a thick mask (extra protection), a telepathic mask (gives ESP for the race), beautiful mask (increases charisma), etc...
CosmicGerbil I like those ideas. There is a script in the ToME libary here http://modules.t-o-m-e.net/script.php?id=23 that allows you to give items to monsters. I reckon that script could be modified to give a chance for a monster to become your companion.
Fury and I have got a couple of masks in our games (crystal mask and terror mask). However they use the helmet equipment slot. I think a new equipment slot could be coded with the T3 Engine.