If the priorities change as suggested here, then not much reason to fly DL or NW if you are a CO Plat and vice versa. Hard for CO Gold and CO Silver to get upgraded - so a DL or NW elite would have no chance at all. So why make the upgrades reciprocal. It should likely be grouped within elite status - then fare.

If a new tier above Plat is added and DL and NW do not add the same - then the best the tier could do would be to have some SWU or other specials. Maybe just call it Plat Plus - and Plat Plus is just ranked within the Plat category for upgrade. NW does have some special benefits for over 75K flyers on high fares - maybe it would be something that that. Say to earn elite above 75k only high fare categories get EQM.

Otherwise what you are going to get - is FT'ers doing lots of mileage runs to get the top tier to get upgrades on cheap tickets over regular Plats on high fares. Now - with the top tier being Plat - the higher fare tickets get the priority. Yes it is just another level - but it the level gets too high so business travelers fall down to regular Plat then the current fare category ranking goes out the door.