I think that Riftmaker can be a viable alternative to Jak'Sho, but as far as I've tested this build, and I've been playing around with it for a long while now (before Jak'Sho came out, I would use Sunfire Aegis as the mythic) Jak'Sho is actually far stronger on Mordekaiser than Riftmaker.
Mordekaiser as a tank becomes a real Raid Boss and still has a ton of damage.

I max W first since it raises how much healing I have and how low its cooldown is.

By the time W is maxed, I already have Jak'Sho finished, which gives 20 ability haste + 8 on the runes, that translate to around 22% CDR which means W is on a 6,25 second cooldown.
As there is revitalize in the runes, every tiny bit of increase in the healing is actually a lot more healing, specially when your health is below 40%.

Since my health pool is really big and my resistances are very high, I abuse W a lot in and out of combat as pure healing tapping W twice fast without really using the barrier unless my health is below 15% (to which I start giving preference to the barrier) if I feel I can get away with it mid fight(if the barrier is very charged) or if my opponent is far away out of combat (by the time we get in combat and he damages me back to how my health was before healing, W is back up again), tanking mostly on my own health and sending W to cooldown earlier.
The cooldown timer only runs either when you heal or time runs out.
This gives a huge sustain early in the game, allows me to stay in lane and farm for a lot longer and grow my gold a lot faster.

Also, since I can count on my healing so much more, I can stay in combat way longer with my passive triggered, keeping my enemies taking damage over time which adds up to a lot more than the Q damage increase.

Even if my Q has less damage by the moment other people would have maxed Q instead, I can stay far longer in fights and abuse my opponents a lot more since I can just heal up after fights.
W healing has a much bigger impact before mid game than Q damage, and if I think I need more damage, I just put 1 or 2 levels in Q and delay maxing W a tiny bit.

Maxing W first is heavily underestimated on Morde, specially with a very tanky build where damage on you is already really reduced.

Also, constantly I'll make Spirit Visage as the second item first, instead of Sunfire Aegis, delaying a even bit more on increasing my damage to which raises the healing of my W even more, because you have Spirit Visage + Revitalize rune, meaning you have 2 percentage increases on your barrier and both also giving a percentage increase on your healing that is based off of that barrier that was increased, so it is like increasing you healing with 4 percentage values.

I kid you not, with this build I constantly get MVP in both damage caused and damage taken while having usually the biggest self-mitigated damage on graphs or among the highest depending on the enemies.

Specially as support and on ARAM, maxing W first makes the game very frustrating for the enemies.