The Darkvision spell, by default, can target only one creature and gains no benefit from upcasting (= using a spell slot higher than the spell's level).
You touch a willing creature to grant it the ability to see in the dark. For the duration, that creature has darkvision out to a range of 60 feet.
Since my level 7 moon druid has two party members without darkvision, I asked my DM if he could allow me to cast darkvision at higher levels to target additional creatures; more precisely, one additional target for each spell slot level above 2nd (similar to spells like Charm Person or Hold Person).
For example, if cast at 3rd level, I can target two creatures, while casting it at 5th level would allow me to target 4 creatures, and so on.
Anyways, he did choose to allow it, and I've used it a number of times so far. However, since allowing this spell to be upcast is so intuitive – I was really surprised when I found out it wasn't possible by RAW – I can't help but wondering if there are any balancing issues to allowing the upcasting of the spell.
To be clear, I'm not asking why the designers didn't allow upcasting the spell, though the answer to my question might easily overlap with the designers' reasons.
What I want to know is whether or not any balancing issues exist that should prevent me / my DM (I also DM occasionally) from houseruling that it is possible to upcast the spell.
Darkvision is a very situational ability, so the issues caused by having this ability will probably depend on how important darkness is.
It's exceptionally easy to end up with a party where every member has darkvision (both of my current groups are in this boat!), unless the DM places some restriction on darkvision-having races. Because it's available to so many races by default, and because it requires a spell slot, this is perfectly balanced. If anything, it may be unbalanced against you, as it takes up spell slots.
It could easily be very powerful in a campaign like Out of the Abyss, if most of the party didn't have darkvision, but that's a unique campaign with a literally very dark setting.
Of the races available in the PHB, 6 of the 9 have darkvision as a sense. This is something that is easily available to the majority of players from level one, and therefore, is exceptionally unlikely to create an unbalanced situation.