Anyway, I'm saddened that Hydro isn't played all that often. I really think it's one of the best maps and have been increasing its playtime on our popular server. One idea I had to give it a fresh feel would use a Rocket Arena 2 or 3 game mode. I'm guessing you're familiar with these Quake mods since they were quite popular back in the day. I think the Clan Arena concept would work great for Hydro and require minimal work to accomplish.
Basically the entire map would be opened up, including the spawn areas, to form one large fighting area. All players would be randomly spawned, perhaps assigning certain spawns to each team so they're spread out a bit. Players would then fight in sudden death mode until only one team has players left alive, and that team would get a point. The game would go for 15 rounds or some other large odd number.
The main drawback to this system is that people don't like waiting for respawn all of the time. Rocket Arena 2 had one solution, which was to make spectator areas where players could mess around while watching the action through windows. Obviously this would require significant map updates to make spectator areas and connecting halls/teleporters.
The other better option would be to try forcing players toward one part of the map. This could be done by randomly enabling one of the control points. So, say a round lasted 6 minutes. Give players a short amount of time to just kill each other and then at 4 minutes randomly enable one of the control points. The control points would need to be neutral and would have to cap VERY slowly, perhaps taking up to 1 minute with a single player on it. This would force players to move toward one location on the map or risk allowing the other team to win.
An additional suggestion would be to remove or reduce the amount of health around the map. This would increase the value of a good medic while really making players think twice about rushing in as scout by themselves.
The strategy of this game mode would be somewhat dynamic, and should appeal toward the competitive community. Teams would have to weigh the cost of fighting alone and spread out to prevent CP captures by the enemy vs communicating and meeting in one area and fighting as a team. Individual class strategy would be affected since heavies would be extremely useful but move very slow for a map as big as Hydro. Lightweight classes would have to avoid enemies in many cases until they find teammates to stick with, which also promotes teamwork.
I hope you like this idea even if it's not realistic to make the change. I know RA2 and RA3 were insanely popular back when I played them, so perhaps it would do the same for Hydro.