Ciaran said:
Amplify Spell increases caster level, while Heighten Spell increases spell level; I'm not sure why you are choosing Heighten Spell for purposes of comparison. Could you clarify?
Because it's the nearest current analogue.
As to Potent Spell, increasing spell level by 1 for an additional +1 DC seems a very heavy price,
This is what Heighten Spell does.
This doesn't make any sense to me. No matter how many metamagic feats you apply using Metamagic Synergy, you only reduce the modifier by 1. So applying a +1 level metamagic feat would result in... +1 level, for no benefit whatsoever. Or am I misunderstanding your point?
Yes, though you do have half a point. Suppose you have MS and apply Still Spell to a spell. Under your system, you can now apply any other +1 level metamagic feat for free, because with MS, you get to deduct 1 level from the total, whereas with mine, you don't.
Example time:
Consider a Stilled Magic Missile (2nd level), and a Quickened Magic Missile (5th level).
With your Metamagic Synergy, you can apply any metamagic feat (e.g. Silent Spell) that raises the level by 1 without increasing the cost, so you'd end up with a Silent Stilled Magic Missile at 2nd level (1 (Base) +1 (Still) +1 (Silent) -1 (Synergy)), and a Silent Quickened Magic Missile at 5th level. That's far too good.
With my suggestion, the former becomes a 3rd level spell (synergy does not apply), but the second stays at 5th level, because Quickened costs more than +1.
It's still very good. Perhaps too good. You can still effectively apply a +1 level metamagic feat like Still or Silent for free when using a high-power metamagic feat. I'd be tempted to restrict the feat further by excluding those metamagic feats which only raise the level by one, but that would make it very complex.
Perhaps it might be better to ditch the synergy concept and replace it with something like this:
Metamagic Focus (Metamagic)
You are particularly apt with one metamagic feat.
Prerequisite: any metamagic feat.
Benefit: Pick one non-Epic metamagic feat you know (the base feat) that costs +2 levels or more. Reduce the cost of that feat by 1 level.
Special: This feat may be taken multiple times. The effects are cumulative (if applied to the same feat). You may not reduce the cost of the base feat below +1 level.