Unfortunately, Larian Team doesn't post on this forum, except occasionally to respond to bug reports. So you won't get your questions answered officially.
Unofficially, my best guesses are:
1.) We'll most likely go to BG in act 2 and it will probably be bigger than cities in Divinity 2 (if only because Larian's size has increased). We see it in a dream sequence, and we're heading that direction in Act 1.
2.) There aren't* runes in D&D 5e, and there's been no indication that Larian plans to add them to BG3.
*The Rune Knight Fighter subclass is based around using Runes, but it's not from the default 5e Player's Handbook and so is unlikely to be included in BG3.