The Nuclear Mage

[Pippin]

Assuming (as I forget) arcane fusion is natively 5th level, making it a Sanctum Spell would cause it to count as 4th level when cast outside the designated Sanctum. But that also makes it take a full-round action to cast for modifying it with a metamagic feat, unless that Arcane Mantle thing fixes that.

To most people, including myself, Sanctum Spell changes the effective level only, it doesn't change the spell slot.

A sanctum spell uses a spell slot of the spell's normal level.

As for the full-round action thing, I believe that Arcane Spellsurge should cover it.

[Segev]

To most people, including myself, Sanctum Spell changes the effective level only, it doesn't change the spell slot.

I'm in the camp that says, "it is what the rules say, but probably isn't what was intended" regarding the trick, so I bring it up in discussions of the RAW. Since you don't believe it works that way, it wouldn't at your tables, which is fine, though. I doubt most DMs would permit these shenanigans, unless they were looking for an excuse to permit the stunt they enabled, anyway.

I certainly can't think of anything else that wouldn't be at least as equally questionable; I can't imagine any rules being written that actually say, "Make this spell a level lower than normal, so it can be used with effects that require lower-level spells." And that's about what it would take to be more explicitly non-cheese in any such interpretation.