Maybe as a new game mode?
I think dartz's idea for this could work well as a new gamemode. In fact, I think there are multiple ways to do this.
3 or 5 "control points." where the spawn changes location, and with each control point, it gets closer to castle.
I do think that the game length for this game mode should be shorter than the current one, and possibly with longer maps (so that the control points do not end up too close to castle). However, no specific time for each control point.
Another option could be that
there is a specific time that the each control point must be hit. It could be something like:
1st Control Point: 20 min
2nd Control Point: 15 min
3rd Control Point: 10 min
I think this would vastly change the game, and it would be more focused on who can get all control points first, rather than castle health.