I think, it will be interesting to make GM mode as a kind of game in the game.

E.G., Game Master have some GM points. He spend this points to change envirnment, quest, monsters etc. Different changes cost different amount of points. And he earn this points for differen activities.
Further in text GMP = Game Master's Points

E.g., some rules:

Common
Click to reveal..
- GM has skill tree, which contains some upgrade for his abilities and new features (type of creatures, scripts etc).
- GM gain xp equal to xp gained by player in his quests. GM gain one fourth of xp gained by player out of his quests.
- XP allows to use new features, scripts, monsters. But to use them, GM actually spend GMP.
- There is some boundary amount of GMP. If GMP decrease below this boundary, it will regenerate in moderate temp.
- Every 30 minute, players can judge GM's actions (voting). Thus GM get rating, e.g., from 1.0 to 5.0. If GM get rating higher than 3.0 he gains some bonus GMP. Vice versa for rating lower than 3.0 he suffer from GMP penalty
- If GM get 1.0-2.0 rating three times in a row he will be kicked from game session (or make voting).
- Any time players can vote for GM be kicked from game session.
- I think, it will be interesting to make some type of GM global score with high score list. After every game session with random players, GM will earn some global points. GMs steady loose some global points every day. The higher score the more points they loose, thus they must play in regular manner to achieve the top of high score list. But anyone can easily have some moderate score without daily games. It will show that they have DM expirience. GM can also get some score points for playing with friends (very low points).
- The higher GM global rank, the more features he can use from the start of new game.


For battles
Click to reveal..
- GM can take over control any one npc in any encounter for free. Players see that. Some GM skills can improve npc in battle (e.g., more health or improved level of ability).
- GM can spend some GMP to take over control every npc in battle. This ability has cooldown and cost moderate amount of GMP. Some skills can reduce cooldown time and cost.
- If GM behave active in such battle, He get xp up to 100% of xp gained by player for killing monsters controlled.
- If GM behave passive in such battles (do no damage for players or use no skills), he get low xp (up to 0), and can suffer from GMP penalty.
- If GM win battle, he gain some additional GMP (but no xp). Thus GM has profit to fight strong: xp for loosing, GMP for winning.


For quests
Click to reveal..
- There are some types of quests activities, e.g., random encounter, ambush, riddle, time rush, side quest, etc.
- Different activities has different cooldowns. It brings more variety.
- Activity cost depend on number of elements used to make it.
- GM can't set amount of xp, players gain for his tasks.
- GM and players gain equal amount of xp during the quest.
- GM can delete quest/objects and refund half of spent GMP.
- GM gain GMP for players get xp in his quest. GMP amount depends on quest variety. E.g., let there are only three sources of xp in certain quest: x1 - expirience gained for battles, x2 - expirience gained for finding certain item, x3 - some another sort of expirience, a1, a2, a3 - some sort of coefficients which depend on type of xp source and player's levels. Let GMP = x1*(1 - a1*x1^2)*x2*(1 - a2*x2^2)*x3*(1-a3*x3^2). Thus, if GM want to make big quest, He should make it with different types of tasks (kill, bring, steal, interrogate, etc). And one complex quest allows to gain far more GMP than several small ones. GMP amount will calculate during quest walkthrough, Thus the more far player advance, the more GMP will be received.


GM's hideout
Click to reveal..
- GM has its own training area, to try out different quest for free. He can save assets and then transfer them on common map. Area's size depend's on GM's level.
- GM can invite players to his hideout. They can do anything they want (GM is some sort of God here), but they won't get any xp, gold, gear etc. What happens here, stays here =)
Player can go back any time.
- GM can once per day teleport players (or some of them) to his hideout to play his own quests. Due to, GM has no cost for placing assets here, He can prepare bloody dungeon with blackjack and hookers several crazy poet mages. GM can't change anything while players in his hideout, but they both get xp for quests. GM can place gears, treasures, but players won't be available to get it outside the hideout. But players will get some rewards for quests completing (random generated without DM) and will save it outside.
- Players can leave GM's hideout after 5-15 minutes after arriving (depends on GM level). Players can leave it after die. DM can teleport players back. And always players can vote for kicking DM.