One thing. It's not confirmed if Flemythal had urthemiels soul in her at the time solas absorbed her because it liked like she was pushing something that looked exactly like what she took from kieran through the eluvian before solas got there.
True I should have said possibly Urthemiel's soul because 1) It's not always saved if Morrigan doesn't perform the Dark Ritual in DA:O and 2) The Developer's note only confirms that Solas is given Mythal's power/essence, but her godhood (which I assume is immortality) is given to Morrigan. So we know for sure that part of the magical soul essence going through the Eluvian is at the very least Mythal's "godhood." It may also be Urthemiel's soul if the Dark Ritual is performed as well.
However, I think it makes more sense that whatever power was to be gained from Urthemiel's soul was given to Solas in addition to Mythal's power, since they are both willing (but somewhat sadly relunctant as the scene conveys) participants of the transfer. It seems the point is to make Solas/Fen'Harel as powerful as possible so he can do whatever he feels needs to be done.
It's also possible that Urthemiel's soul could be a wild card and counter to Fen'Harel and it was transfered to Morrigan instead. I guess it depends if Solas knew of the events that occured with Morrigan, Keiran, the Inquisitor and Flemeth in the Fade or if could sense the soul inside Flemeth in the epilogue.
@Crescent_Moon
Although I think your theory is far fetched as the shield link is very questionable (Which I think is just a copy of Darkspawn shield skins from all installments, not to mention it looking serpent-like is very subjective), but the likeness/art style of Solas' Tarot card and the Shartan mosaic are very similar to near identical. Them being one of the same is at least a little quesitonable though as we speak to Shartan's ghost in the Temple of Sacred Ashes.
That doesn't mean they weren't the same entity, as the voice/look differences could be technical (of course) and Shartan's ghost may indeed just be an echo/spirit created in the temple. I forget the details around the nature of the spirits there.
Wouldn't it be interesting if Shartan, the leader of the rebelling elven slaves under Andraste, was Solas/Fen'Harel the old elven god of rebellion and with Andraste, successfully freed the elves from slavery? If true, I wonder if based on his personal history and his position on gods in general, that Fen'Harel left for the Fade once more after the Dales were granted to the elves, only to find out much much later that the Dales were retaken in a second Exalted March. It may also explain why Solas is hesitant to teach and help current elves in Thedas by basically saying what's the point? So that a simple man rises up only to be cut down by overwhelming force?
Maybe, based on the events surrounding the first and second Exalted marches, Solas/Fen'Harel/Shartan plans to be a little more extreme in his actions to free elves from human oppression, by securing any future elven freedom from human interference forever. Thus the need for the orb's power and other extreme measures like taking Mythal's power. Of course he doesn't need to be Shartan to feel this way, but it sure would make his possible fight for elven freedom (if that is his true goal) a lot more personal throughout that ages than it already is.
Again Solas/Fen'Harel trying to secure elven freedom could be very bad news for the current humans of Thedas. At least for human influence over the world.