I'd like to hear from other players about what they'd like to see in Favored Soul prestige classes as well as what they expect Turbine to implement. Personally, I have no PnP experience so I have no idea what might be drawn from to implement FvS PrEs in DDO. However, going on Turbine's designs so far, I would not be surprised to see three initial FvS PrEs - one focused on healing, one on offensive casting, and one on melee.
- Melee
I would guess a melee PrE would be similar to Kensai, as they both focus on a particular type of weapon. I wouldn't be surprised to see a +1-2 to-hit and damage with a diety's favored weapon at each rank, a +1-2 bonus to a skill or two (maybe concentration?), and maybe some kind of special effect at tier 3.
As for what I'd like to see - I'd like to see melee focused favored souls close the dps gap a bit with pure melee classes. I'm not necessarily saying that should happen, it's just what I think would be cool after a couple minutes of speculating. It's certainly possible that a large dps increase would simply make favored souls too powerful.
Another possibility is adding effects like Superior Devotion/Potency when diety weapons are equipped. I say that simply because I get tired of switching between scimitars and scepters in the heat of battle to try and get a heal off or throw a blade barrier. This is not necessarily ideal for a PrE benefit as it can easily be solved through smart itemization, and may even be rendered impotent later on with the introduction of more powerful effects on better gear.
- Healing
No idea here, as there are currently no healing PrEs in game from which to draw. This would probably grant additional spell points at each level, a +1-2 bonus to skills (heal?), as well as a percentage increase to the effectiveness of heals. Also maybe a percentage increase to the effectiveness of wands/scrolls? I don't have much speculation of my own here.
- Offensive Casting
Once again, we have little currently in game from which to draw. Bonus spell points, +1-2 to skills (concentration?), +1-2 bonus to spell DCs, and maybe a percentage increase to damage? I've got little of my own speculation to add here.