There should be a filter for a variant of Cities&Knights where both progress cards and Developement Cards from the base game are usable. I always found it “unsemmytrical” that the 5-6 Player for C&K dont introduce any more progress cards and/or VP. You still have the standard 3 metros, and 2 vp cards(1 from science and 1 from politics) and the same number of merchants despite their being more players. You also have the same 54 progress cards(18 form each branch) despite their being more players and a higher risk that progress cards run out although still super unlikely.
I think a simple but very interesting variant they can introduce would be to combine the developement cards and the progress cards into one game so players can use their ore, wheat, sheep to purchase a developement cards and go for army or collect some of the many VP cards in the Development deck. Along with still having the option to use that ore, wheat, sheep to summon and activate a knight to go for Protecting the colonies from those savage barbarians VP.
There a plethora of details that need to be worked out like how the progress cards interact with dev cards and vice versa. I feel as if the best solution is that for the most part they remain independent but no doubt there are exceptions.
Dev Cards: Still only once per turn. If Dev card is used, progress cards can still be used as normal but other dev cards cannot be used that turn. And cannot use the same Dev card that was purchased this turn.
Knight - Still hidden until used and does what regular knight in the base game does. Has no influence over map knights and competes for Largest Army the same way.
VP Cards - Needs to be revealed once obtained as there are cards like Master Merchant, Wedding, Sabatuer that interact with it.
Road Building - Can be used regardless of if Road Building from the science progress cards has been used or not but not if the other RB from the Dev deck has been used.
Year Of Plenty - Gives 2 Resources as normal but cannot be used for Commodities.
Monopoly - Can be used regardless of if any monopoly from the trade progress cards has been used but not if the other monopoly from the Dev deck has been used. Can only Mono resources and not commodities. Can mono all of the resources and not just the 2 per player.
Science Cards: Alchemy, Crane, Engineering, Invention, Irrigation, Medicine, Mining, Road Building, Smithing, Printer all should work as it normally does. Note RB from this deck can be played even if RB from Dev Deck was played. Note Smithing upgrades field knights and has no bearing on Dev Knights.
Trade Cards: Commercial Harbor, Master Merchant, Merchant, Merchant Fleet, Resource Mono, Trade Mono all should work as they do in the base game. Note Master Mechant also accounts for the VP cards pulled from the Dev deck as they are now public.
Politics Cards: Diplomat, Warlord, Spy, Intrigue, Sabatuer, Bishop, Deserter, Wedding, Constitution should all work as normal. Not Spy cannot peek or steal Dev Cards. Note Wedding and Sabatuer also account for VP cards from dev deck as they are now public.
There may still be some details I have to work out but I belive it is possible to make the Progress and Dev decks coexist for a more in depth and strategic Cities&Knights game. If anyone has any questions or something I missed, please reply to this comment.