In Heroes By Trade there are eight classes, each of which has themes associated with it both in terms of mechanics and flavour text. Of course the most successful games are ones where the mechanics effectively reinforce the flavour, so I am trying to make that happen. One thing I want to be careful of though is making classes too narrow in scope. I remember what it was like in old DnD, where if you had a ton of enemies about you had to have a wizard because fighters could put out decent single target damage but were completely hopeless against groups. I want to have themes, but I think it is good if people don't specifically have to rely on a particular class to fill a particular role.
AOE effects are the biggest thing here - in both DnD and HBT they iconically belong to the wizard. My wizards use fire, ice, and lightning to blast huge areas and blow up tons of enemies at once. That is their shtick! However, I don't want them to feel like they are pointless when there is a single enemy to fight, nor do I want everyone else to check out when there are swarms of dorks.
My solution thus far has been to give everyone access to multiple target attacks, but to give most classes only a couple of options, and often to make those options fairly niche in application. The Champion *can* AOE, but they can only do so while standing in the middle of a pile of enemies, and usually only by Grabbing all the enemies in the process. They are meant to be tanks, so that is how their AOE effects play out. Wizards on the other hand have a dozen different ways to AOE and can tailor their AOE to the situation at hand. They will generally have exactly the sort of AOE the situation calls for which tends to make them the best at it.
The trouble I was having is that I wanted every class to be able to defend itself, AOE, have some mobility, deal extra damage, and more. Also Powers are divided into ranks and if a class only has a single AOE ability that is found at Rank 13 that isn't any use to someone starting off who can only use Rank 6 Powers! Balancing the need for individual flexibility with the structure of the Ranks was possible, but I ended up losing out on flavour. There were just too many boxes to tick and I ended up with a bit too much homogeneity.
The structure I had was two Powers at each Rank from 5 to 13 for a total of 18 Powers per class. By covering all the bases I generally only had 7 or so really thematic Powers per class, and 11 more generic ones. My new plan is to keep all the Powers I had already built but add in 4 new ones to each class within the existing Ranks. As long as all 4 new Powers fit the class theme really tightly I will raise my 'on theme' rate to 50%, which should be enough to make classes feel really different.
It is really tricky to walk the line between having classes be too narrow and forcing people into strict roles, and having classes be too flexible so that everyone is similar. I think I am getting closer to the right spot in the middle, but it is a tricky thing to do.