I've also considered the replacement possibly, except possibly going one step further. Make ALL 6th- and higher-level spells into incantantions. The higher-level spell slots could then be used for more lower-level spells, improved by metamagic feats or not. This would be best for the kind of low-magic campaign that appeals to many people.
Another idea I've played around with is using incantations to get around the spells per day limitation. With this option, any spell you know or have prepared counts as an incantation that you know. You could then cast that spell without using up a spell slot by making a successful Spellcraft check every 10 minutes (60 minutes minimum).
In either case, I'd probably use the base DC because spells already fall under the category of pre-packaged magical effects. Of course, this was all just theoretical and with no actual playtesting.
Another idea I've played around with is using incantations to get around the spells per day limitation. With this option, any spell you know or have prepared counts as an incantation that you know. You could then cast that spell without using up a spell slot by making a successful Spellcraft check every 10 minutes (60 minutes minimum).
In either case, I'd probably use the base DC because spells already fall under the category of pre-packaged magical effects. Of course, this was all just theoretical and with no actual playtesting.