
Tarantula |

Spell Slots: The various character class tables show how many spells of each level a character can cast per day. These openings for daily spells are called spell slots. A spellcaster always has the option to fill a higher-level spell slot with a lower-level spell. A spellcaster who lacks a high enough ability score to cast spells that would otherwise be his due still gets the slots but must fill them with spells of lower levels.

Pizza Lord |
I think the only real trouble would come from metamagic. For instance, if a 3rd level spell could fit in 3 1st-level slots, you could have someone try and place a metamagicked 3rd (raised to 4th) spell in four 1st-level slots, or in 2 2nd-level slots, whereas they would not normally have that capability to affect that spell until 4th-level slots were available.

Ruanek |

I think the only real trouble would come from metamagic. For instance, if a 3rd level spell could fit in 3 1st-level slots, you could have someone try and place a metamagicked 3rd (raised to 4th) spell in four 1st-level slots, or in 2 2nd-level slots, whereas they would not normally have that capability to affect that spell until 4th-level slots were available.
If this was implemented I don't think it would work that way.
If 2 level 1 slots can be used for a level 2 spell, then maybe 2 level 2 spots could be used for a level 3 spell. So that would take 4 level 1 spots for a level 3 spell.

Tarantula |

I agree it could be a worthwhile arcane discovery. Like the inverse of split-slot.
Something like:
Combined Slot: Once per day when you prepare spells, you may treat two of your open spell slots as if it were one spell slot that were two spell levels higher. For example, a 9th-level wizard can combine two 3rd-level slots into one 5th-level slot, preparing Cone of Cold in the 5th-level slot. For all purposes, the single higher-level slot is treated as that higher level (so the combined 5th-level slot used for a Cone of Cold has a DC as if it were in a normal 5th-level slot). This discovery has no effect on cantrips. You may select this discovery multiple times; each time you select it, you may split another spell slot when you prepare spells. You cannot combine a slot that you created by combining two lower-level slots. You must have at least 1 spell per day of the level of your combined slot. You must be at least 5th level to select this discovery.