The score of each player is by default the same as the number of frag.
I would like to be able to have the
score = frags - deaths
Having stats at the end of the match is different than changing the score rule.
It is possible to extend the way the score is calculated, included different score for each weapon or having a chain bonus as already mentionned.

Exactly !
I've made a mod for the standard q3a, but I will be far more appealing with e+ mod.
Hope it will be possible !

I see you have all good idea with custom rules to make the game more strategic/balanced.
The force of E+ is to be very customizable. And the use of xp variable can make an unique game mode.
If you have already played Halo, and Halo 3, you known there lot of parameters to manage to make your own ruled match. Take a look at http://halowiki.net/p/Default_Gametypes for more details.
It will be great to make a list of the variable needed to change the match rule in a more strategic way.
I have already proposed one :
xp_deathreward default 0 : will be added to the player score when he die.
Here some from the read messages:
xp_fragchain default -1 : define the number of frags made in one life to make a chain of frags.
xp_fragchain_bonus default 0 :will be added to the player score when he is reach the number of frags of a chain
xp_fraglimit1 default -1 : define the the number of frags to make to obtain associate bonuses
xp_fraglimit1_speed default 1: define the speed factor that applies when you reach the xp_fraglimit1
xp_fraglimit1_life default 100 :define the life limit that applies when you reach the xp_fraglimit1
xp_fraglimit2 default -1 : etc...