We've already given the devs tons of ideas to make buildings more appealing without directly giving %, not sure why they are staying with the same stalemate prices.
% of 150 attack (like AO)
% of absorb all/half damage
-1 attrition upon collection
New units such as pirates that appear as enemy troop until hit
+1 turn in the next negotiation?
Because giving something new and giving another dimension to power creep isn't entirely a good thing - it creates a wider gap between enfranchised players and new starts. Also many of the things you mention would not be universally considered good ideas.
New units they haven't done in a while because they're too much work to make them useful to everyone but not overly distorting. i.e:
- Rogues were distorting and ultimately not a healthy addition. And now they can't take them away or even nerf them too hard or it leads to serious backlash.
- Champions were added in a balanced way but add new work to every new age they create - which has led them to being of varying use. For a good while they seemed determined to make them so bad they just didn't have to worry about them.
- Drummers and Color Guards were added in a safe way. And notably they're not used beyond the first few ages for more than fodder sieges in GvG.
AO crit they've shown regret over as is (not letting it into Arena) - seems unlikely they'll add new sources of it.
A defensive avoidance stat while it would be of interest to me, is indeed another dimension of power creep.
GBG does not need more power creep by letting people lower their attrition
An extra turn in one negotiation would be way too underwhelming.