Here are my thoughts in no particular order.

1. Titles would need to have more benefits than just a shop. If for example you need to be a Duke to buy a RoA/RoM etc then it's going to be much more efficient for one person to be a Duke and act as an intermediary. Why spend 15m to be a duke when you can just give me 10m and I can buy a RoA for you. If on the other hand each rank boost also gave a stat boost (+1 str/dex/con/int/wis/pow/cha) per level I'm pretty sure a lot of people would want to be a Duke regardless of shop for the +5 to all stats boost. If special dungeons did exist they'd need to justify high cost to access them.

2. I'm personally not a huge fan of multiple currencies being required since you're always going to have a limiting currency. Being realistic lets say person runs a raid and everyone gets a worldboss "soul" then it's likely very few will be able to afford the mithril cost. On the other hand grinding for money will also be pointless if you can't acquire worldboss "soul" easily and if you can then it's redundant as 10m is harder to get.

3. Just to give a current implementation would I be correct in saying it's like cider shop with a special currency and items for sale being rare drops. In that case these quests would give the special currency as a reward with better ranks giving access to better shops and/or better discounts. Presumably there would also be a way to buy that currency. It would also mean that people could choose to cash in their rewards or save them up for a better item that costs more.