what you should consider is giving for free or at low cost triple inventory space and add more vanity related items for sale.
and or adding tradeskill inventory space for free and in item shop.
reasoning:
people wont buy more space because even if they buy it, it wont allow to maintain the amount of vanity items, tradeskill items, power items players want to gather and hold.
personaly and trust me its not just me, people create guilds on alts just for the extra free inventory space.
allowing players to have triple space would in long term possibly be more profitable for funcom if and only if you increase at same time offer in item shop of affordable amenities of all kinds.
just from old world armors if sold as vanity in item shop either in bundles or piece by piece or separated by set; i know loads of people who would buy those sets for vanity because first off some of them look realy awesome with great detail and also because if your not a soldier some of those sets cant be used at all.
increasing for free and in item shop bigger inventory expansion items while increasing offer of bind on equip items with account wide option could be very profitable for funcom and interesting for players to spend money on.
as well as adjusting ridiculous prices on some items like the refeat related items, camels and others.
having some new items like equivalent to guild npc merchant, and adding trainer npc spawner without requiring rez pad would be win if at low affordable price with a use once type of item.
give us the conditions to spend more money and we will, if you even add commitment that, that money will be spent developing aoc instead of other fc games people will spent even more just to develop hyboria.