I’d like to share my two point of views on this, on one side as a tech freak and on the other side as a technical manager of a small company with 300+ TYPO3 instances (and I think we can all relate to both to some extend).
PHP 7.1+ is a no go, MySQL 5.7 should not be a minimum requirement, IE 11 needs support.
See below for details and inputs.
As a tech freak, I’d say PHP 7.1 is a must because of the improvements it might bring (and just think of the additional performance!), but also as a tech freak I think that the core should be rebuilt as a ReactPHP-Servlet style standalone-inline-PHP Server able to run by calling
php -S localhost:80, so there is a pretty biased point of view about performance and what TYPO3 (or any software by this standard) has to be about performance over everything. And from this point of view, PHP 7.1 or 7.2 makes total sense.
As a technical manager I see 300+ TYPO3 with still tons of legacy code that is already pretty hard to maintain, update, document and test in an 18 months lifecycle. Don’t get me wrong, I love the 18 months lifecycle because from a sales point of view, every 18 months you get a new product to sell to new customers that is up-to-date by the latest web standards and is fancy and stuff, but from where I stand, I’ll also have to maintain said sold product for at least five to ten years and if I have to include a server upgrade every 3 years in order to just be able to install the latest LTS of TYPO3, either my boss or my customer will get very angry at me because a.) either I’m doing work that isn’t paid or b.) I have to charge at least two extra days for an upgrade that brings him no noticeable added value.
Now, it is true that TYPO3 evolves thanks to the community (which I’d put in the tech freak group) that want’s to progress and use new technologies and have fun with new features and make everything pretty and shiney, but in the meantime, all money that goes into TYPO3 (I’m aware that most of the work done on TYPO3 is not paid for) comes from agencies and companies that need a stable product they can rely on and on the end of the day, these are also our customers and clients that allow us to put bread on the table.
Therefore: PHP 7.0
As for IE, I think really strongly about it, as all of you do, but again, big companies will still use it in 5 years so dropping the support is a no-go.
And about MySQL: I think everything has been said.