I don't have anything to comment on the matter of powerful characters, though. Pretty much all of my characters are around the power level I want them to be already. Even if there's a bit more of room for improvement, it's more on the field of mastery than raw power growth.
Juno, Helena, Hohenheim, Iona and Taura are all so powerful that I barely ever can have them use a tiny bit of their capabilities in any given situation. With those five rule of fun/rule of cool usually precedes anything else when I'm writing.
Ciel is a case of her own. It's hard to even explain the magnitude of her importance in the greater scheme of things. On her own she's not that powerful, tough. It's her summons (Including the skill to summon any of the afore mentioned characters and much more) that make her a real threat in case she join any battle.
Eos (who, if no one noticed yet, is actually Ni-Mu as well), is way more reasonable, unless she has to push things to the point she had on KC-00032. However, that was pretty much a one time wonder and the weapon she used will be soon returned to its rightful owner. I still have plans to use Eos in other future battles, though. Now that she finally found her way, she must continue to walk it.
Diana as basically reached her maximum power already, she's totally in control of her draconic powers and magic. She can't possibly get more powerful, just learn to use them better.
Victoire can't get any more powerful than she already is by any feasible means. She's already in the top five most powerful vampires in existence, trying to climb further up is ranks is considerably harder than it looks like since the gulf between each of them in enormous. However, she'll keep on getting more crafty and using her magic/natural powers in even more diverse ways.
Anyone not mentioned is hardly relevant at all, or a joke character who may or not be a lethal joke character in disguise. *cough* Absinthe *cough*
Essentially, my characters case of power escalation reaching an asymptote (as everyone is realistically supposed to at some point, by the way) getting more powerful is possible, it just takes an impossibly huge amount of time for it, since there's no such thing as "maximum power" if you keep on pushing things further. Except for the first five, whose power is downright off any limits and Ciel whose role isn't to fight at all.