Definitely iPad Pro event.
Mac Pro? likely not. They have publicly stated it won't be ready until 2019.
https://www.macrumors.com/2018/04/05/apples-revamped-mac-pro-to-launch-in-2019/
That said, they could show it off. If I recall correctly, that's what they did with the last Mac Pro design (trashcan model). Demo'd months before it available for order.
Mac mini will be interesting. There is rumormill on it, but inflated price.
iMac is over-due, but they may be hesitant to release, since a iMac i7-8x00/9x00 will cannibalize the starter iMac Pro. Especially if Apple is considerate enough to throw in Vega (debatable, finally seeing Vega lower prices again, still a bit high over MSRP on initial launch, but AMD is still churning out Radeon 5x0's).
As for custom chips. It's interesting that Apple's A12 is apparently approaching desktop levels.
https://www.anandtech.com/show/13392/the-iphone-xs-xs-max-review-unveiling-the-silicon-secrets/4
(bottom of the page) On certain single threaded tasks.
They are already using some sort of custom ARM chip on touch-bar MacBook Pro's.. it would be interesting to say the least, but any custom chips are still likely in the hybrid "A-chip"+intel stage for the immediate future(Secure Enclave, flash/file storage), before they ever reconsider their reliance with intel.