Just want to contribute some ideas of game modes I had.

1. Remake Slaughter mode

Instead of having to reach a number of kills:

One of the teams has to reach 1.000-10.000 points to win (adjustable).

For every 100 damage that you (Player, your creeps don't count) deal to the enemy creeps or minions you get 1 point for your team.

For every 100 damage that you (same) deal to the enemy towers you get 2 points for your team.

For every 100 damage that you (same) deal to the enemy Demigods you get 5 points for your team.

For every 100 damage that you (same) deal to the enemy Citadel you get 10 points for your team.

For an enemy Demigod kill you get 100 points (points are given if the Demigod is killed by your creeps).

Any upgrades that boost tower hp/damage, creep hp/damage or spawn new units for the wave are disabled. All of these are acquired automatically for both teams over time.

Capturing spawn portals is disabled (either remove the flags or make them not capture the spawn portals).

The Citadel deals double damage and has infinite (or a very large number, like 1kk) hit points. You can win by attacking it and getting more points.

Info: Does this mean that getting damage will now be more important than hp? Not really. If you only get damage, then although you won't have much hit points (and points for the enemy team to get), you will give the enemy 100 points if you die (and you will die more often). The same way around: If you buy hp, you won't give the enemy team those extra 100 points for the kill but you won't get points faster than the enemy, so it's roughly balanced.

This game mode will be a lot more fun to play as it encourages going into action and dealing as much damage as you can. Also: the enemy Demigods won't be able to stay near the crystal not to lose (like they do now), as you get more points for attacking towers and the citadel.

2. Remake Dominate mode 

Instead of always getting points, teams will now only get points when there's a difference in flags captured. For every extra flag that the team has in possession, it gets 1 score every second. If both teams have an equal number of flags, score is not given to any of the teams.

To win, a team must get 500-2000 (adjustable) score.

The Citadel deals double damage and has infinite hit points (the game is not about this).

Also, a new feature: Captains

The first Demigod to make a kill on both teams becomes a Captain. When the first kill is made, a 2 minute timer starts and the killer doesn't become a captain yet. These 2 minutes are given for the second team to try and get a captain too. If the second team makes a kill too, then the timer is removed and both killers become captains. Otherwise, if the timer ends, the first killer becomes a captain while the second team is left without one (it can still get a captain if someone kills someone).

If a captain dies 2 times in a row without killing anyone, he loses the captain status (and it can be acquired by another player or by the same one with a kill).

What does the Captain do?

The Captain captures flags at double speed. If both an allied captain and an enemy Demigod (not a captain) stand near the same flag, then unlike normally (flag meter doesn't move), the captain will start capturing the flag at normal speed. A captain vs 2 normals will result in the flag meter not moving.

So, basically, a captain counts as 2 demigods when capturing flags.

3. Remove Fortress mode, add Survival mode 

Fortress mode is strange and pretty boring. Survival mode is not (and is pretty common).

Both teams start at their spawns.

Both bases are surrounded with enemy towers so it's impossible to get out. Towers have infinite hit points and kill from 1 hit (can't be dodged or blocked).

Portals in the shops are disabled.

There are spawn portals near the center that spawn enemy creeps for both teams (light creeps for the dark team, dark creeps for the light team). Creeps come from several (usually 3) directions.

Ally and enemy Demigods have shared vision, so players see each other.

The citadels are removed.

Gameplay: Creeps become stronger and stronger with time and new creeps are added to the wave from time to time (priests, giants).  Maybe a game length setting should be present? It shouldn't be possible to survive for too long.

 The team loses when all of it's Demigods die.

Also, there could be an option to play survival without an enemy team. For this, starting a game with players only on one team and on Survival mode should be possible. In this case the players should spawn in the center. There should be more spawn portals when only 1 team is playing (creeps could come from 4-6 directions). The enemy creep/tower color should depend on the team they chose when the game started.

If only 1 team is playing there should be a timer in the center (and yes, I know there's a timer in the scoreboard). Generally, it should be a bit harder if only 1 team is playing in the center.

Final thoughts: 

Modes like these would be fun to have. It would be interesting to watch what strategies people develop for Slaughter and Survival modes (and the Dominate mode too). Watching Survival replays could also be interesting.

And it would be a lot more fun playing Tournament (since the game modes are pretty different).

Maybe someone could make a mod to implement these game modes?

Numbers can always be changed.