Usually the height of the PIFA determines its matching efficiency.
However, since PIFA is commonly used as a low profile antenna, it is not desired to increase its height for BW enhancement.
Other common techniques are perturnations in the radiating plate or different taperings.
Can you elaborate on your PIFA? May be there is a more suitable solution.
The resonance of the slot will occure at slot length=lambda/4or2. u can first design the simple PIFA and then adjust the lenght near to ur required resonance frequency.
I hope in turn it will enhance the BW due to the combined affect.
Bandwidth enhancement is usually required at low frequency because lower the frequency more narrow the bandwidth.
Therefore bandwidth enhancement required for low frequency design e.g. slot, combining two near resonant frequency etc.