It is a given that new mantles can be created. this is proved by the fact that we have Santa Clause - a clearly recent invention - as a mantle held by Odin.
What might such a new mantle look like. I can speculate on some aspects
1) Mantles are tied to belief in some way That is why so many mantles are tied to old gods. Belief is also tied into the power of the Fey as evidenced by Mab's encouragement of the brothers grim fairy tales. Belief is also tied into the ability to manifest into this world - as evidenced by the Oblivion war sidebars that show up in short stories. However, many examples of mantles continuing after belief has waned, so not clear how important belief is in maintaining the power of a mantle.
2) Mantles represent people -- the are personifications of a belief. A mantle is not "gift giving", it is Santa Claus.
3) Mantles require volume. Lots of people have to believe for a sizable block of time. There is no direct evidence of this, but it is logical. The world is not buried in Mantles after all, so there must be a hard threshold that must be met.
4) I wonder if a mantle can be deliberately created -- for example by a Norse god wanting to shelter some of his waning god-power. He might break off a piece and "empower" a mantle for something meaningful in this new age - like Santa Claus. That act might be required or it might simply speed the process up.
Speculation on possible modern mantles* I think I can rule out super heroes, movie characters, or similar creations of pop culture. People simply do not believe they are real.
* I think I can also rule out most conspiracy theories where belief is widespread and intense from the mantle creation possibility because they typically describe vague hidden forces rather than specific individuals.
* You might get a religious figure (the pope, etc), but these are not normally seen as "more than human" in that sense at least.
* You could have the "hidden special person" type mantle. The mysterious figure in hiding from the modern world. This could be the Dali Lama, Elvis, or Marylyn Monroe. All it takes is a lot of people believing "they are still out there in hiding". But you have the same problem as with the Pope. Simply because they are hiding does not make them a figure of great and mysterious power -- which is what mantles provide.
* I could see "Murphy" as a mantle that is created. Murphy as in Muprhy's Law, not Karin Murphy
. Many people believe and it is a personification of somewhat great and mysterious power.
Other ideas?
And before anyone mentions it, i will acknowledge that it is possible that the total "pool of power" available to mantles could be fixed. New mantles could simple be some portion of some old obsolete mantle refashioned into a new mantle. After all, there are almost certainly mantles so old that they have lost meaning (and thus potentially vanished...). However, for purposes of this question it does not matter if mantles are created new - or out of the "dust" of old obsolete mantles. The key point is that "new" mantles can exist.