There's a fundamental relation between antenna size and maximum bandwidth of electrical small antennas. If you are already scratching this limit, there's no chance to increase the bandwidth without adding artificial losses.
Hi,
I would suggest the addition of some parasitic elements close to your radiating element , or to etch slots whether in the ground plane or in the front in order to enhance the bandwidth.
Good luck!
Hi,
As you might know the antenna design is an 'error and trial ' process so it is hard to say which ones you must try, instead I would recommend you to start with the most used slots such as the strip, T, L..etc
for the dimensions they are usually taken lambda/2 or lambda/4 .
Good luck!