Come to think of it, specialization probably won't be the primary driving force behind alts. The tomestone weekly caps will be.

Regardless of how you play with the crafting system, a weekly cap like that combined with a specialization system necessarily doubles your progression as a whole if you have alts, when compared with someone who has to narrow his focus.

For example suppose I can choose 3 specializations and I invest my weekly tomestones into buying recipes for one of that specialization. Now I do it on two characters, I effectively doubled the number of specializations I have, and doubled the number of top-tier recipes to which I have access. Since I only ever need to put money in gold sinks for one character, I effectively double my earning capacity.

So a "true" omnicrafter in 3.0 would have his own FC (use the FC chest to transfer items and gil), have all specializations spread over several characters, and have access to all top-tier recipes, cap tomestones everyweek. By the time the no-alt crafter max out tomestone recipes on one craft, the omnicrafter have got everything.

The required investment is leveling a combat class to 60 on each alt, and all crafts on all alts to 60. With a lv50 master crafter already available, it's just a matter of time to produce HQ items for leve turn-ins. Since all alts earn leve allowances in parallel, the progression speed is only capped by how quickly one can produce those HQ items.

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This does mean, though, that a closely-knit FC that distributes specializations among members will derive that same benefit without anyone running alts. So in a way it supports a community effort while still leaving a backdoor (and a rather powerful backdoor) open for the true omnicrafter.