There is a recent superb article about variant games by Arathorn:
here
I'm still thinking how fare are the bonuses for the variants. Based on what is suggested in the Arathorn's article:
AW - 1.0-2.0 level up
Defiant - 0.5-1.0 level up
Fast or slow moving - 0.25-0.5 level up
5CC - 0.25-0.5 level up
5CC Conquest - 0.75 level up
OCC - 0.25-0.75 level up
Tactless - 0.5-1.0 level up
Obviously, 5CC and OCC bonuses depend on map size.
Also, very rough estimation based on very limited experience:
RBC exploits - 0.1-0.25 level up
RBC honorable rules - 0.25-1.0 level up but may be up to 2.0
Mad-bax's modification of rules - 0.1-0.25 level up
Also, a crazy idea: How to discourage "closed" teams. If all team members have more than average 200 (may be 150 or 100 or 300 or 250) "Global Player Ranking" points per player at the date the game starts, all bonuses are either gone or severely diminished, lets say, become 2 or 5-fold less. This may stimulate incorporation of unranked or low-ranked players in the teams and would make a "dream-team" less profitable enterprise.