Spell Combat

Spell Slots

A higher Spell Slot can be used for a lower level spell, as in the PHB. However, 2 lower level Spell Slots can also be exchanged to replenish one of the immediate next level.
This can be chained together, ex: 4th: 0/1 left 3rd: 1/2 left 2nd: 2/2 left
Character spends 2 2nd level slots to get 1 3rd level slot (putting them up to 2 3rd level again they then spend both 3rd to get 1 4th level. Now they have 0 2nd or 3rd level but 1 4th level.

Scrolls

Any character can use a spell scroll, using their INT modifier as a spellcasting modifier. If the character is not a spellcaster and the spell is a higher level than half the character's level, they must make an Arcana check with a DC of 8 +(spell level x 2). On a failed check, the action is still spent but spell fails.

Stealthy Spells

A character can roll to attempt to cast a spell covertly, at DM's discretion and depending on the components. (Ex: a spell with a somatic component would involve a Sleight of Hand check.)

Detecting Spells

If the character perceives a spell casting, the spell's effect, or both, the character can make an Arcana check with reaction or action. The DC equals 15 + the spell's level. If the spell is cast as a class spell and the character is a member of that class, the check is made with advantage.