A player asked me if they could use their 17th-level Mystic Arcanum (Warlock class feature) to select the 6th-level spell "Mass Suggestion" cast at 9th level (to always make the suggestion last a year and a day).
I am unsure how to answer this (final verdict), as Mystic Arcanum specifically states:
At 11th level […]. Choose one 6th level spell from the warlock spell list as this arcanum. […] At higher levels, you gain more warlock spells of your choice that can be cast this way: […] and one 9th-level spell at 17th level. (PHB p.108)
So my take on this is that they are very specific about which spells you are able to choose (spells that are on the warlock spell list, and there, they have to be n-th level).
Am I wrong? Is there any rules citations which support what the player asks?
Spells don't take on a higher level until you cast them using a higher level spell slot.
From page 201 of the PHB, under "Casting a Spell at a Higher Level" (emphasis mine):
You are not casting the spell when you pick it for your Arcanum so no, you wouldn't be able to select Mass Suggestion as a 9th-level spell when you reach 17th level.
Rules designer Jeremy Crawford has also unofficially confirmed on Twitter that unless the DM allows it, you can't choose higher-level versions of lower-level spells for Mystic Arcanum (e.g. choosing a 9th-level version of a 6th-level spell at warlock level 17).