> 1. Could it cause any problems(side effect) ?
Some I2C device connected to the bus may not work correctly.
> 2. How to increase the fall time to meet spec ?
Off my head, we usually apply 2.2K ~ 10K resistor for pull-up.
Depending on the value of this resistor the fall/raise time will be changed.
Lager resistor makes the transition slower and smaller resistor makes the transition faster.
The calculation described in the I2C specification may be a good reference.
17 Application Information
17.1 Slope-controlled output stages of Fast-mode I2C-bus device
(I'm referencing The I2C-BUS specification Version 2.1 Jan-2000 from Philips Semiconductors)
We try to modify resistor for pull-up before , It can improve rising time, but it is not useful for fall time.
Please give us some suggestion to increase the fall time.
Then how about adding some resistor serial to the SDL/SCL pin,
so that current drain time will be increased?