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So the general idea is to have a limited skill pool. To simplify the concept lets give players 500 points if they want to pick up an ability they're going to have to spend 5 to 50 points for that ability. Abilities vary from: New crafting ability, new combat ability, easier to get things from NPCs, and similar.
to allow for some easier balance and familiarity there are some traditional skill trees/ability trees that players advance through. The starting node costing the 50 points, and branching further around/down the tree costing 5-20 points. With no limits for how many/few starting nodes you can choose, how far you want to delve into trees, or which trees you can combine.
The largest concept that I want to discuss is the fact that it's a completely shared pool between combat/crafting abilities. And how to balance it so that it's interesting, has trade offs, and fun.
some stats: Crafted items a baseline: 1.0 power. Random loot drops: .8 of crafted items. Epic/ancient/story/raid items: 1.5 of crafted items. Crafted item in hands of maxed out crafter: 1.5 - 3.0 of baseline. Well crafted item in hands of combat character: 1.0 to 1.3 of baseline. Epic item in hands of maxed out crafter: 1.5 of baseline. Essentially refering to the "power tier" or advantage of the item itself but in isolation not taking into account the abilities of a character, team, or similar.
What a character gets out of combat skill trees: New abilities pretty much identical to what is seen in normal MMOs, and ways to modify their abilities as they advance around/down the skill tree. The only real differance is that the tree will be the only way to get non-basic abilities, and there being more ways than typical to modify them.
What a character gets out of crafting sill trees: Initially the ability to craft the item, then as they progress down the the tree they gain the ability to spend time to tinker the stats(where well crafted items become 1.0 to 1.3 power by min maxing). Also as they advance down the line there are some optional some not where they will modify an items stats while holding it, Essentially removing negatives and/or amplifying positives. Which is where with proper tinkering a crafter can end up with an item 3.0 of the baseline(double the raw power of an epic item).
End results and goals from this
Pure crafter: gets some pretty large rewards by applying their trade. But they're going to be using baseline abilities which means no form of crowd control, and no means of really changing the tides of battle. Essentially aside from basic intuition of knowing when to defend, or attack the fate of the battle isn't in your hands "you have no Aces". But you'll have a large static tank built, some nice raw damage as well.
Pure combatant: Get some pretty large rewards by having all sorts of tricks. Sure you won't have the raw static tank, or brute force of attacks. But you'll be able to temporally buff yourself/allies, have some active tanking abilities like shields/damage sharing, have some healing abilities, have access to CC, and similar. If you play "perfectly" the pure crafter will be alive for a long while but the battle will eventually fall in your favor. However the pure crafter will have far fewer mistake that they're capable of making, meaning it won't be a sure and easy fight.
Mixed: the person decided they like a particular type of item/stat/skill so focuses on ways to get them better through crafting trees. They won't have as many ways to change the flow and pace of a fight, but when the fight is in their favor they hit(defend/heal/whatever) very hard. Essentially this is where it comes down to the player becoming closer to a "class" that focuses on particular aspects.
Note: any crafted item will get the bonuses rather than just self crafted items. All items(even epics) have decay attached to them, meaning that even pure combat people will use crafted items for simple battles.
Note: Tinkering is something that is meant to be able to be done indefinitely, though eventually the player will just fall into cycles where they just undo old work for a change and similar. However tinkering skill/leveling/whatever advancement is still better than "new item creation" even if they are just retreading old ground. Item spam is silly.
Practice doesn't make perfect, practice makes permanent.
"At one point technology meant making tech that could get to the moon, now it means making tech that could get you a taxi."