The 2018 BMW i3 will be unveiled at the Frankfurt Auto Show in September - which is not far away, so you should at least wait until next month. Production is set to begin in November with delivery to dealers staring hopefully early in 2018.
I struggled with the same question about waiting, but decided to order a 2017 i3 Rex in May. I took deliver in late July - barely 2 weeks ago. It is a wonderful car, and very nimble in city driving. The main reason I did not wait until next year is that the 2017 has decent enough range, and there is currently a very good government rebate in Ontario of $13,000 on the car and $1,000 on the level 2 charging station. As this incentive has made EV purchasing in Ontario very popular, its success may lead to its demise as it will have served its purpose of increasing EV sales.
Given the rapid pace of development of EV and battery technology as well as the pressure that the Tesla 3 will put on the competition, anything you buy today will look obsolete in 1 or 2 years anyways. If it meets your needs today, it will meet your needs in 3 or 4 years when the next model will surely look a lot more interesting than either the 2017 or 2018.