Prime32: Simply saying "All spells have a DC based on the level of the spell slot used, including with other metamagic" would close so many loopholes.
I don't think that's a good idea. Then you can Quicken spells and get an extra +4 on the save DC. Let's just make metamagic more uber, shall we?
I think it would be better to have spells cast using higher level slots (although NOT because they were adjusted with metamagic) to be counted as spells of that level. For a completely unoptimal example, let's say a Wizard prepares Fireball in a 5th level slot. This sets the Fireball as a 5th level spell for all relevant effects (i.e., save DC, Spell Turning, etc.). Another Wizard prepares an Empowered Fireball in a 5th level slot. This is treated as a 3rd level spell for all intents and purposes other than spell slot used, being Empowered. This looks much better to me. That way, you don't have DC 15+Int Mod Fireballs dealing 10d6 damage and DC15+Int Mod Fireballs dealing 15d6 damage in the exact same slot. The latter would have to be used in a higher level slot still for that extra effect.
Yes, lets. Because frankly most metamagic sucks donkey balls. Almost no one ever actually uses any metamagic aside from Extend, Sculpt, and Quicken without some kind of "reducer" like Divine Metamagic and Incantatrix levels. The original idea behind metamagic was supposed to be that it was
actually worth using without reducers. Clearly that's
not the case.
So yeah, I'm all for having the DC set by the slot used. This actually would benefit people who use it "the old fashioned way", but not those that use cheese like DMM (Quicken). Throw out Heighten Spell entirely, and have the spell's DC determined entirely by the slot used for it, regardless of whether it has metamagic applied to it or not.