I was just thinking about soul trap uses and i came up with an idea. What if when you soul trap an monster or npc and you used the gem to summon that person to fight for you for the time being pending on what gem you used like a lesser gem would have a shorter time then a grand gem. What do you guys think.
Well I'd like such idea it also can have such features like in below
Daggerfall have really interesting Advantages&Disadvantages system for enchanting one of features was Soul Bond,
all creature souls lend additional effects on enchanted item
The more powerful of a creature, the more powerful the item can be. Certain creatures have extra advantages and disadvantages forced upon you for using them to enchant your items. If your item breaks, then the creature is released, and will attack you.
All souls have not only enchant power but also own cost and name
http://www.uesp.net/wiki/Daggerfall:Enchanting_Items
http://www.uesp.net/wiki/Daggerfall:Enchantment_Power#Soul_Bound_Items
For Oblivion there is mod Unnecessary Violence thats allow use great number of new and axed features from previous games, one of features allow thrown soul gem with captured soul into enemy or surface around Soul or Ghost of creature will be released and attack closest actor to him and then disappear after some time it can attack both enemy and you so be prepared.
Since Oblivion have Black Soul gems thats allow trap soul of NPC, you can used the gem to summon that person to fight for you also.
There can be some additional features done like
Named filed soul gems what combine cost of soul and cost of soul gem so it will be easy search for desired soul or for collection in houses.
About of time calculation
I think if soul trap spell goes to summoning and all souls have own levels there can be more calculations steps like
Power of soul versus quality of soul gem and sum of enchanting and conjuration skills = ghost life time
or more hardcoe one if there will be check versus conjuration skill for time when creature will be under control of master and not be hostile for him,
if time such time is greater then ghost life time you it will be under your domination and will not hostile for you and attack only enemies if not be prepare to run or fight for your life until soul will disappear or be destroyed.
In such cases Conjuration will have more interesting options as well we able conjure in such way almost all creatures in game what can be soultrapped, we receive powerful ally thats can be more powerful then conjured daedras not count in summon limit and not need special spell effect for conjure,
but not for free it will cost us filled soul gem of various quality which break on impact, powerful soul what can be used for enchanting, and creature what soul we want release for first must be killed for soul.
As part of dynamic magic Conjuration will have different mechanic instead of summoning duration for conjuration will be used
Permanent summon thats decrease maximum of magicka until be destroyed or dismissed, decrease is large enough to feel it impact on gameplay and can have actual limit like 50 magicka for Scamp or like percentage cost 25% of maximum magicka for permanent summon.
Channeled magicka drain for support of powerful conjured summons what cannot be summoned in permanent way because of high magicka cost for permanent summon or great powers of summoned creature
In both cases summon duration depend on used magicka so there no need anymore of the same spells thats have difference only in duration which affect spells cost now Magicka will work as duration limit so if you have enough large Magicka reserves but novice conjuration skills you can summon permanent ally for self only one and thats what have low requirements unless you have skill buff and found more powerful summon spell just don't have enough skills to undertsood it or have equipped enchanted item thats allow cast such spell if you have enough Magicka reserves or item has enough charges to compensate percentage of costs.
Conjuration skill will increase summoning limit, such limit will based on conjuration skill levels
Novice can only summon one creature
An Apprentice can summon one creature from Apprentice level spells or two from Novice level
A Journeyman can summon one creature from Journeyman level spells or two from Apprentice level or three from Novice level
An Expert can summon one creature from Expert level spells or two from Journeyman level or three from Apprentice level or four from Novice level
A Master can summon one creature from Master level spells or two from Expert level or three from Journeyman level or four from Apprentice level or five from Novice.
So having summoned powerful Master summon need much concentration on binding it Nirn and control it willpower such concentration will not allow summon additional creature, but If you are Master of Conjuration you can summon instead of on master level creature summon one expert creature + one Journeyman + two Apprentice ones.
Summons can be more intelligent not all will mindless rush in combat when see target certain can accept commands from caster and attack marked target or even have tactic customization in dialogue, not all of them can do this certain will not understood you or refuse such commands but certain
like Dremoras and Golden Saints can, but they can also disobey with certain commands for example Dremoras can accept change in tactic from protecting Summoner from melee enemies and switch to attack of ranged enemies but will not agree be your beast of burden, refuse bring item to summoner, or give him their equipment, they can also say something to master in such cases, while summoned novice Scamp can not only mock your enemies in combat since they cannot understood tactic and will rush in combat but also have simple utility services like bring item for master search for ingredients around, need to understood they cannot do complicated tasks but simple things like thats they can, just need watch for them because they can bring havoc also in area
Flame Atronach can also have some additional traits like light up area around them and work like walking torches or lanterns
Clannfears will not understood you an be wild or can service as summoned mount
Additionally
Character level will affect level of summoned creature and it powers and equipment
Summoning of daedras and undeads in public places can be described as suspicious hostile or crime act so guards and town NPC can react on your unholy actions, thats work for permanent summons also so dismiss your troops before go into public place, alternative there can be official allowance in form of document in inventory thats you as ranked member of local mage guild can cast such spells without crime bounty unless they are not harm anyone.
So conjuration of summons will be much more interesting then constant spam of cast button for always conjure one summon what always mindless rush into combat switch to wrong targets and even don't do any strike because duration for spell is out too fast.