I think it would be wise to include a donation progress bar for leveling up kingdoms much like the one we have for guild donations. This will allow people to balance their guild donations with their kingdom leveling instead of having to wait to donate to a guild until a particular kingdom is maxed out.
As of now:
To max out a level 9 Kingdom I need to stockpile 40k Gold. However, if I am part of a guild and donate a portion of that stockpiled gold, I would have to continue playing the game to regain the 40k Gold to be able to max the kingdom.
With the donation progress bar, an empty bar would slowly fill up with each gold donation until it reaches 40k Gold. In other words, instead having to stockpile 40k Gold, I could donate any gold amount towards my kingdom’s Lv. 10 progression. This way I would be able to manage incremental donations to the guild and my kingdom leveling progression.