I forgot to check, but did the vanity card at the end reference Ms. Susi's passing? I'm not sure the turn-around time, but I figured that would be the first place they'd mention it.
While others, such as Melissa Rauch, can do an imitation of the voice, I think it's time to retire the character, much in the way the Simpsons retired Mrs. Krapabble (though with maybe more in the way of working into the plot). It'd be weird hearing Mrs. Wolowitz knowing the actress who originated the role died.
It's also a good time, with Howard being married for a few years and the current plot line with Stuart, to have her die and Howard have to deal with her death.
If they need to fill the hole, maybe Raj's mother, now that she's split from his father, can move to the U.S. and pester Raj, giving Howard a chance to see a mother/son relationship from the outside.
Rest in peace, Ms. Susi, thank you for the laughs.