As a quick suggestion, I'd typically start with the following objective in mind:
Assuming you're going to permit all the standard PH and DMG cleric domains, what's the minimum number of deities you can implement in your converted mythos which provides the players with sufficient options to play the characters they want to play?
Taking the Norse pantheon as an example, let's give it a shot. I'll also try, where possible, to have both a good and a neutral option available to broaden the range of playable cleric characters.
Knowledge: Odin (CG, representing the knowledge he gained from sacrificing an eye), Mimir (N, the beheaded god of wisdom)
Nature: Frey (NG, fertility and the harvest), Skadi (CN, goddess of hunting and skiing / winter?)
Light: Heimdall (LG, guardian of the rainbow bridge), Dagr/Delling (N, god of the day/dawn)
Life: Balder (LG, beauty and rebirth), Freyja (N, goddess of love and fertility)
Trickery: Bragi (NG, music, bards, poetry), Loki (CN, mischief)
Tempest: Thor (CG, thunder and storms), Niord (CN, seas and wind)
War: Sif (NG, goddess of skill-in-battle), Tyr (LN, war and duty)
Death: Hel (NE, queen of the underworld)
Once the basics are covered, I would typically add in any other important gods. For example, Frigg (wife of Odin) might be another option for Knowledge, and Forseti (god of justice) might be another option for War. And then, if required, add secondary domains to some of the deities if justified by their theme. So, maybe Thor also offers the War domain, and Frey also gets the Life domain. This makes the converted pantheon a little more interesting, and not just a straight list of 1 good + 1 neutral deity per Domain.
Just a suggestion, anyway. I've used this "bottom-up" approach (i.e. figure out the PC needs, then modify the gods to suit) many times, and it seems to suit my group well.