Navi is obviously coming, and it will be 7nm.
The RX590 is clearly a stop gap card - it's exactly the same as the 580, just 12nm vs 14nm for the 580 which allows them to pump higher clock speeds through it. Board partners can take a 580 pcb and drop in a 590 chip and they'll have an RX590 card. It's obviously not a long term product. OCUK have been clearing Vega 64's and 56's at close to cost price
Lisa Su has said that they need to clear inventory, so I wouldn't expect Navi till at least March.
As for performance, it's expected to have 40 compute units (vs 64 for the Vega) - with the smaller die size and move to 7nm they'll be able to pump more clock cycles through it.
They'd have to do over 2,000 mhz to match or better the 2070 I've heard - they've managed to push the 590 to around 1600mhz vs 1300-ish mhz for the 580 on a die shrink of 14 to 12nm, so that doesn't seem unreasonable with a new (or newish) architecture and a die shrink to 7nm.
The move the 7nm is significant as it will have a lower tdp and allow for higher clock speeds.
All they're aiming for is vega 64 (which they've already managed)/1080/2070 speeds.
As for cost - figure around 590 price plus extra for the more expensive Gddr6 memory.
Al Nvidia have achieved with the 20xx series cards is to match the price/performance of the 10xx series cards.
If they matched the gains the 10xx made over the 9xx series, then the 2070 should be the 2060, the 2080 the 2070, and they should be priced accordingly, at 10xx prices.
The 2070 is really a mid range card, being sold at high end prices - all amd are trying to do with the first Navi cards is match what should now be mid range performance, with a massive die shrink and new architecture.
It's Nvidia's prices that are unrealistic, not amd's - they're not trying to beat a genuine £500 card, because the 2070 shouldn't cost anywhere near that.
AMD are widely rumoured to be developing Navi for the PS5 and the new Xbox, which will give them development money.
Those consoles are expected to achieve 4k @ 60fps - so that is where amd will ultimately be aiming with Navi (but unlikely with the first versions).