The Vancian style of magic system has been around since the begining. Nealry from the begining there have been spell casting variants. Look out world here is my version.
When a cleric announces his actions and intent to cast instead of declaring..."I cast cure light wounds". Clerics will now declare..."I call upon my deities favor and pray for cure light wounds".
The Game master will have to track spells perday by level for the cleric. With a simple piece of scrap paper and 1 minutes time this can be preped before the game begins. Just check off each spell slot as it is used.
The GM decides if the deity grants the spell, denies the spell or sends and alternate spell.
For GM's who are unsure of the deities whims or those who desire a bit of the trusty random chart.....
1 - spell denied
2-9 lesser or alternate spell granted. (granted alternates should be of the deities main focus)
10-17 Spell granted as desired by cleric.
18-19 Spell granted with a bonus (wisdom bonus added to damage or healing..durration etc as GM desires)
20 Bonus spell (higher level granted or grants an extra spell slot for the day)