Honestly, I believe that what ESAR's is saying (the core concept) would work rather well. It bases your experience (SP) on the experience (actions) you have in battle, with the KEY DIFFERENCE TO THE SYSTEM WE HAD AT LAUNCH BEING: "If in a party, every participants SP is
calculated together and split evenly."
This way you don't have someone getting 50 SP while others get 300 SP, it's all split evenly amongst those that performed at least 1 action during the course of the fight (so on easier fights healer only needs to top off the tank once and the the healer gets SP as well, or they can simply cast a nuke or debuff or something).
In groups, this makes it so that you want to fight mobs that take more than 2 seconds to kills since you want everyone to get their shot in, but you don't have to do the crabs/efts/whatever that take forever to kill either, you can move on to the next mob if your camp has the mobs/respawn rate to support you.
This also means that if you are geared to the teeth and can 2 shot a mob that it takes someone else of the same rank 4 shots to kill, you will have to kill twice as many for the same amount of SP, which will take you slightly longer since you have to find more mobs / switch targes and all that, but this means that maybe you should fight something a little harder that does take you 4 shots to kill instead of 2. (though the trade off for killing more in less time is that you get more drops, so it could be worth it depending on the group/player).
And being geared to the teeth you still have the advantage when fighting for progression (you know, that content that is coming

)instead of just SP grinding, so the incentive to min/max isn't totally lost.
ALSO
This actually caters to the player base that SE is going for (casuals) that won't be geared to the teeth. Well, not that it caters to them, but that it gives them roughly the same SP/hour (unless they pause between actions/mobs for whatever reason) that the elitists players recieve. And of course hard core players will always level faster simply because they have more time to devote.