One could argue that a concentration spell creates a kind of arcane connection between you and your spell, which explains the potentially infinite range at which you can maintain concentration, but has some other slights problems
Would you allow a magic item with the "keep concentration" effect modifier (+5) to keep concentration from any distance ?
E.g.: a wand of wall of stone, who create a stone wall with concentration duration, +5 keep conc. effect modifier.
On how many instances would you allow such an item to keep concentration ?
E.g.: Same Wand of stone walls, but with 6 use per day. You could create up to 6 walls, one after the other. Would the item keep concentration on all 6 of the walls ?
Would you allow a magic item that duplicate the effect of "maintaining the demanding spell" ? ReVi (Gen)
Would you allow it to have the "keep concentration" modifier ?
Same with many uses, so that it could possibly maintain concentration for all spell cast by the wearer of such a magic item... ?
I personnaly think that the MuVi restrictions about such magical effects (effect of MuVi in a magic item can only affect magic in that item) should also apply to some ReVi spells.
I would appreciate to know how you deal with such things =)
I would increase the difficulty of your concentration check the greater the distance you travel, kinda like "out of sight out of mind", since an item can't do concentration rolls it would automatically fail if it traveled to far from the effect. Similar with creating the effect more than once, it is more difficult to cast additional spells while maintaing effects, so the difficulty level rises and the item fails to maintain the 1st effect.
So, to make sure I understand the question, have the item effect similar to maintain the demanding spell made with a duration of concentration and the effect modifier "item maintains concentration"?
Yes I would.
I don't think that these are game breaking effects, I don't believe that the benefit of house ruling them away would be greater than the pain of having yet one more house rule.
To take my guess at the history of the restriction on Muto vim spells from functioning on spontaneous magic I'll take you back to the dark days of third edition.
In those olden days wizard's boost was a spell that did not have to be form specific. Wizard's boost also worked on spontaneous magic.
What this meant is that anyone who knew wizard's boost was likely to cast all of their spontaneous magic spells 5 levels higher than they would have if they didn't know wizard's boost. That's akin to +10 on your roll. It's a really really good deal. Some minmax minded player (such as myself) would have their magus get their hands on the spell and after about three game sessions when the other players saw how much ass it kicked they'd get the spell for themselves.
When fourth edition came about the folks responsible fixed what they interpreted to be a problem and lots of people were happy with the change (myself included even though it stole one of the best tricks from my vim specialist Bonisagus).
I haven't seen the same sort of "everyone's got to have this spell" effect with any of the rego vim stuff in my games. So I personally am not eager to implement more house rules to nerf the characters (players tend to not enjoy those sorts of rules).
as is every thing in this thread...and a good deal of the answers to most questions asked.
There is nothing canon about items and concentration so I figure a non-game breaking suggestion may be more helpful than not saying anything unless I have a page to quote from the book which, as with any rpg, is a set of guidelines and not hard rules.