I agree to an extent. As much as I would like to see Eilistraee in DDO, I really would like to see more Eberron in DDO and less FR. We have an entire continent of Khorvaire and the only places we've explored there are Sharn and an abandoned facility in the Mournlands. I would rather visit Thrane or Karrnath.
Eh, Tiamat the Mesopotamian character is quite a bit different than the character that WoTC has the copyright to. Sea serpent vs 5 headed dragon goddess who has a red, blue, green, black, and white head and rules over chromatic dragons. In Eberron the only difference is that she corrupts dragons whether they are chromatic or metallic although she has an easier time with chromatic ones. Whether or not they are the exact same character I think is purposefully left up to interpretation. It's just one of the many things WoTC likes to leave open so DMs can use it as a way to move players between different settings.
Tiamat from Eberron/FR vs Takhisis from Dragonlance is a similar but more complex can of worms.
Something related to this that I thought was interesting is the characters from the Feywild expansion. The Prince of Frost and Titania are both characters that I am familiar with from the Feywild in the Forgotten Realms, but I've heard discussion that the Feywild in DDO is from the Eberron setting yet different from Thelanis. I really don't understand how things tie in together in that regard but I guess that's part of the whimsical nature of the Feywild.