Perhaps cut the current XP given per tick in half, and replace that half with a hybrid of what we had before. You get the normal "per tick" xp, but you also get xp for being near kills that occur, or for scoring kills yourself.
Having a hybrid XP system would eliminate some of the lameness/leechers by making depending ONLY on "per tick" XP extremely inefficient. Thoughts anyone?
The down side is, with a hybrid system with per tick+near killz and/or score, the PRO frontal melee will have 100% xp, and the archer/noob/flanker will have 50%; it could lead to too much frontal attacks and lame xp progression for certain people.
With the actual XP earning system, the only down side I find is leeching (and perhaps not enough reward for killing): but leeching is easily spottable and righty bannable, and one could say the best reward for scoring kills is helping your team to have more XP via the multiplier.
Thus I think the actual xp system is the best I heard of for the moment, and should only be tweaked via increasing or reducing the "per tick" time or the amount of xp by tick.
Although, the gold system could be separated from the xp system by adding other factor to the "I grant thou gold", but idk if its worth to consider even a subtle change in the actual xp/gold system.
Idea from reinhardt : assisting system sounds good, an opposing system could penalize TK by withdrawing some xp and/or gold.