Wizards would get this because they are what they are, monster tamers.
Obviously if something like this was introduced to Wizards then other classes should obtain some alternative spell / passive or feature to improve their ability, I would go on the lines of something like this;
Warriors
Due to the nature of this game Warriors have been designed to take heavy damage, so for PvE sake it would be a great idea to introduce some form of Taunt (to keep aggro).
Wizards
Monster-index + Taming points (Improves PvE aspect)
Taoist
Healing on this class should be looked at in more detail, I would suggest that healing on another player should have a chance of healing the Taoist for a certain amount of life over time (so in a group hunt the Taoist would single heal a warrior and on chance receive a HoT).
Assassins
Not entirely sure what to give these guys, but since I'm covering the PvE aspect here then maybe something like Feint...Where if the Assassin has aggro they can use this spell and loose any aggro.
Archers
Never played one of these before so I'm unaware of their current skillset, however make something to improve their mobility...like traps (freeze,ice,poison) that they can place down.
Again, not all of the adjustments made to classes have to be visual or a spell...Passive spells and such are just as good. Chance on hit or Cast additions to items would be another way to improve diversity, making each player specifically different but balanced.
To solve PvP, I could only assume that the use of a new stats (Resilience, Hit Rating). Only problem with this is that items already dropped would have no chance of taking on a stat and i believe a wipe would be needed for this anyways? Unless you could upgrade the DB.
People killing on Mir has totally different meaning to it than other games (its either for fun or greed, no actual gain). I think the problem that lays in Mir is that their are no real allocated areas or reward for actually PvPin (other than getting the boss you want), so taking a page from many games i would suggest bringing in a whole pvp system with actual pvp maps and pvp based items (Tier's).
These can / could be bought using only PvP Points so obviously the better at PvP you are the more PvP items you can get, and hopefully with the use of the two new stats making it a more balanced game (Resilience will increase your Ac,Amc by small % and lower hits landed, Hit rating has an adverse effect).
Without going into tons of detail and doing proper calculation I feel this would be a good start...but then again, Mir is Mir and people love it for what it is....and seem to hate change (better or worse)