They should really give Luvdisc an evolution now that it's niche is gone. Though I'm half-convinced Alomola started out as a Luvdisk evolution but was changed when they decided not to include old Pokemon during the story section of B/W.
I've often wondered this after I read that Ken Sugimori stated Basculin was created late in development when they realized there were few new "standard" fishlike Pokémon in Unova. For me I've always wondered if the reason they suddenly had this issue was due to them deciding no old Pokemon and thus they could only use new Pokemon and thus decided to cut Pokemon like Magikarp and Luvdisc. It would also make sense given the previous gen had introduced a lot of cross-gen evolutions and it could have been a continuation. But that's more speculation on my part.
As for a new evolution for Luvdisc I definitely would like to see it as even if the evolution wasn't that great it would make it more memorable and to be honest the ability to hold evolite could make it slightly sturdier,
And everything about that was a mistake really. Honestly, can they just retcon that? And throw in Pachirisu and Emolga while they're at it? Emolga is EXACTLY what I imagined a Pachirisu evo to be and it's a huge waste that that design and concept is for a regular standalone species instead of a Pachirisu evo.
Do I think they will retcon it so Luvdisc evolves into Alomomola? honestly no.
Would I personally like to see it? yes.
Could they do it? yes but it would add some challenges. Firstly they'd have to deal with the gender ratio being different (but to be honest they managed it with Azurill.) There's breeding which they'd probably have to add an incense similar to what they did with a lot of the gen 4 baby Pokemon. There's also the issue that unlike the baby Pokemon Luvdisc can breed and as such they'd have to deal with both an evolved form and a pre-evo being able to breed and produce different Pokemon (which is doable but a challenge). They'd also probably have to treat it as some discovery made that they didn't realise they were connected . Lastly they'd probably have to introduce an evolution method that wasn't possible before given these two Pokemon have both been obtainable in the same games before being connected. But they could easily have some new item added in SV that makes it possible.
Would it be useful? Yes IMO. As mentioned above it would enable Luvdisc to hold eviolite and it would give you more reason to use either as you'd get Luvdisc early on and then rather than replacing for something stronger you have the option of evolving it and keeping it around. Thematically they also work quite well given that Luvdisc represents romantic love connected to couples and then Alomomola is more a nurturing healing love like a parent (it even has mom in the name) and could offer some new move options for both. Sadly I don't think it will happen.
As for Pachirisu and Emolga I'd like for them both to get evolved forms but to be honest I don't see Emolga as enough of an upgrade on Pachirisu for me to be an evolved form. Plus statwise they are very close(and they aren't strong already like Scyther and Scizor). I'd prefer them both to get something to evolve into that are significantly stronger stat wise.