If it's going to end up being released a substantial amount of time before the successors to the other 8th gen consoles then yes I agree it'll be a perculiar situation. Usually, I'd say the generation each console gets filed under depends on the timeslot of it's release together with the consoles it mainly competes with.
It's obvious that Wii, XBox 360 and PS3 are in the same generation, because they were around for roughly the same span of time and competed with each other. Same with the PS2, GC and XBox or PS1, N64 and Saturn. It's disingenious to say they aren't in the same generation because Nintendo skipped the 2nd. Those were the groups of competing consoles no matter how you look at it. Where would that end anyway, do the generations before the PS1 count for Sony or is it in the 4th, XBox in the 3rd and Nintendo in the 7th? The 'power' question is irrelevant.
The easiest way is to say each successor heralds in a new generation, but then there is the case of the Atari. The 2nd gen is a strange one anyway, it's basically split up in two parts and it probably would have been better if it was two generations, and the NES would have been the 4th instead (and so on and so on of course). There is however something to say about everything before the crash being part of the 2nd gen. The Atari 2600 outlived it's successor the Atari 5200. So 2600's 2nd generation of consoles outlived the apparent 3rd generation of consoles with the 5200 in it, relegating it to a '2.5th generation'.
Now on to WiiU and NX, with the rumours going around right now about it being a low-end system releasing next year, if it's a home console, I'd say for now it's still 8th gen because it's going to be competing with PS4 and XBox One. There's no way it will then have a significant enough life to have the PS5 and XBox successor as it's main opponents. If WiiU would live on and ends it's life together with the other 8th gen systems including NX, then there's something similar to the 2nd gen. WiiU, XBox One and PS4 would remain '8th', NX '8.5' and all their successors 9th. If WiiU would be discontinued now, I'd say it would fit more with the 7th gen, considering the original Wii and the other 7th gen entries are still on the market, similar to the Atari situation. NX would be Nintendo's 'true' entry into the 8th gen. I think the only way we can keep WiiU as 8th gen and have NX as 9th gen is if the latter releases late enough or has a long enough life, while outlasting WiiU obviously, to primarily compete with PS5 and the XBox successor for a significant amount of their lives.