Even if Jaina is a potent Archmage I think she is not powerful enough to become the new Aspect of Magic without being imbued by someone/something more powerful.

The point is :
Is Magic able to find it's new aspect by itself ?
or
Considering that the only powerful being related to Malygos death is Alexstraza, is she the one who should transfer Malygos essence to the new recipient (thus becoming the new Aspect of Magic) ?

In the second case Alexstraza would probably need some advice from Krasus and I think that the Red Archmage would probably be tempted to have someone close from the Kirin'Tor (or at least friendly towards them) becoming the Aspect.

I doubt it. Remember: Aspects are not "natural". They were created and handed their charges by the Titans themselves. There's no reason to have Magic work itself out an Aspect, since it "worked" pretty much without one prior to the Titans' reordering of Azeroth. And while the Titans are extremely powerful beings, able to engineer the Dragonflights (and hence the limitations that prevent other flights to take charge), I don't think they reordered reality itself to require Aspects.

(a few years ago, it could be argued differently, but the Titans have been steadily retconned from Supreme Beings into supreme beings, no caps)

As it was alluded earlier in this thread, the events when you kill Loken in the Halls of Lightning, with Loken firing up a Titan beacon (the sci-fi classical upward firing beam of light into space) clearly indicate that the Pantheon is being notified. A petty revenge from Loken ("you brought me down? well, good luck explaining why a bunch of old-god-perversions, undead and/or alien immigrants was doing good destroying Azeroth infrastructure stuff to them")

I think the Heart of Magic is going to be what transfers the power to a new aspect. I'm not keen on the idea of Arthas raising Malygos, its the same as he's done twice previously and we've yet to fight Sindagrosa. It might not even necessarily have anything to do with the relative strength as a combatant, more that they seek to protect magic and the planet. I wonder though if a human is chosen would the blues follow? Did Metzen shoot down Rhonin as a possible? If it was going to be a human he seems the most likely candidate, especially with his direct involvement in the Nexus war.

For that reason, shouldn't it exactly be the opposite? I am not sure if the Blues would like to follow someone who helped kill their former aspect after all. Someone more neutral like Me'dan or Jaina would be a better fit. That is, of course, if dragons think like us humans; someone earlier said that the reason there were no Blue rebels to Malygos because they may not think like us after all (Possibly a mild form of hive collective? Really, how many rebelling dragons are there anyway? None?).

I think Rhonin has an important role to play already as the leader of the Kirin Tor, why shift him away from it?

The new leader of the BD will depend on whether Blizzard wants to have them remain hostile to us, or whether they want BD to have a seat at the Wyrmrest Temple again and join us in the fight against Arthas. It is possible that The Blues get split into two factions, one which supports our efforts (led by Kalcegos maybe) and another led by the new BD leader that is still working against us.

As much as I was for supporting the idea of Jaina for the new Blue Aspect, remember that Dragons live for a Very Long Time. Would we have an aspect that would die again in even 100 years?

I can see it happening as a means of ending the Nexus War by hopefully bringing together both the Blue Dragonflight and the Kirin Tor under a single banner. Besides, I'm cynical enough to believe that Rhonin Garystu's portfolio is only going to increase. Imagine it:

Rhonin
<Leader of the Kirin Tor, Aspect of Magic, Lich King, Lord of Ulduar and Master of All he Surveys>

Who else was thought dead and fell into a pit? Kael'thas.

Aegwynn was at least something like 800 years old and she was "merely" the Guardian of Tirisfal.

Kael'thas didn't fall into a pit, his body was left on the floors of Tempest Keep. Which most certainly didn't plunge into the depths of Outlands...

Interesting observation. Perhaps the runes put servants of the blue dragonflight under the same kind of compulsory obedience that members of the blue dragonflight normally operate under at all times? Maybe when the new aspect is chosen, the servants of the blue dragonflight get their free will back and can rally under the new aspect, but the members of the blue dragonflight can't, and remain enemies with no ties to the aspect of magic from that point forward? Maybe "magic" will have no associated dragonflight at all at that point? Dunno.