My group and I are playing with a classification of spells called 'Augment' spells - primarily 1st level spells who's casting time will be a swift action and will apply some sort of benefit or enhancement to the next spell cast provided that it is cast the same round and can be cast as a standard action.
These spells might replicate a metamagic feat, they might create an ancillary lingering effect themed off of the original spell and so forth... the purpose being to give higher level prepared casters more options with their lower level spells.
Generally speaking, if an Augment spell replicates a metamagic feat it is of a level equivalent to the level boost of the metamagic feat (i.e. an 'Extend' augment spell would be 1st level and a 'Dazing' augment spell would be 4th level).
I really think this opens the doors to a lot of interesting options and I'd be interested to hear the thoughts of others.
Yeah I ran a similiar idea a while back -- if you had the metamagic feat you could spend a spell slot of the spell level that matched the increase from the feat to augment a spell cast on that round -- it took a swift action to do this but all in all it worked out alright.