We need to have a digivolving Gatsumon to Meteormon
They should really do all of the Royal Knights. Will already have a couple of them taken care of, though I think Kentaurosmon would be fantastic. Lastly, they really need to do a better Leomon and redo Guardromon. Those two would be awesome.
Not really digivolving spirits, but it be cool if they did all of the Hybrid Digivolving Digimon for season 4.
Impmon to Beelzemon is at the top of my list. But I’d also like to see:
Meicoomon to Rasielmon/Raguelmon
Lucemon to Lucemon FM or SM
Dracomon to Examon
Hackmon to Jesmon
Zubamon to Durandamon
Shoutmon to OmegaShoutmon
Examon is generally treated as a jogress between two normally Dracomon lines. So if they were feeling a ton of love for him, they could go the combiner route, but I think it's more likely they'd rather put that work into a more popular digimon and just go directly from Dracomon.