Frankly, I don't think the regular iPad will be updated in March 2019. The difference between the early 2017 and early 2018 iPads was quite substantial. The difference between the early 2018 iPad and a possible early 2019 iPad would likely be too small.
Wasn't there like two and a half years between the iPad Air 2 and the early 2017 iPad? I think the difference between the current iPad and an early 2019 iPad would be too small for Apple to update it.
WWDC 2019 is probably not the right moment for an update either. So at the earliest, it'll be September 2019.
(The iPad Pros obviously will not see an update before late 2019. So it might be a year without any changes to the entire iPad line-up)
Just my guess.
Now, the reason this interests me is actually more in consideration of the future of the iPad Mini. Apple still sells it. Would they just drop it in March 2019 (without saying anything about it)? Would they replace it with a Mini version of the early 2018 iPad? (Considering the rumors by Kuo that the Mini might indeed see an update). Perhaps they drop or refresh it late 2019 when there might be a new regular iPad?
Come to think of it, I'm now guessing there might not even be a March 2019 event. (2013, 2014 and 2017 didn't have one either.)
Things like the new Mac Pro might be more of a WWDC thing, rather than an update to the iPad Mini.
Guess I'll be buying a Mini 4 then... In late 2018, for about a late 2015 price...