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I suggest having timers in the uper left part of the screen that can be clicked to start a countdown for when an objective monster such as the Ancient Golem and the Dragon are would respawn.
This would be an advanced UI option that is defaulty toggled off so as not to clutter the game for new players for whom the information may not be helpful.
Many times, a player knows that an objective has been taken by the enemy but does not and will not see the monster dead.
Similarly, not every player knows the exact timing for when an objective is taken, and putting that information in-game would help elimate some of the need for Google searches to improve game performance.
This would make keeping track of objective respawn times easier for experienced players while helping to teach new players the timing and value of these objectives.
Tl:dr- Timers for objective respawn times in the top-left of the screen that you click to start as a toggle option- defaultly turned off.