So the other night me and a guildy were discussing what would be nice to provide more uses of the acid/lightning line of wiz/sorc enhancements.
I made the comment that the energy substitution line would do this without having to add new spells.
Now I can see the issue that making a metamagic that would prompt a drop down to be interesting to code for the game, I came up with the idea to make it when choosing you decide what energy to specialize in. So if I wanted to spec lightning I could take the feat and choose Energy Substitution (Lightning), then when ever I activate the feat it would swap damage to lightning based. When the spell was not on the spells would function normally.
In pnp I remember that the cost was equivalent to extend. I dont see this costing 10 extra spell points and not being considered OP... However I can see this being approximately the cost of empower or maximize as this would allow a lot of freedom for casters and a little more viability.