I'd also say the 2 techs that are interesting less for their density aspect and more for longevity are flow batteries and liquid metal batteries. Both would solely be used for grid storage (Flow MIGHT be used for home storage).
Flow batteries are interesting because they have an very long cycle life and their storage capacity is based on how big a holding tank you have (fill the tank, get more storage). They are less interesting because they require mechanical pumps to operate.
Liquid metal batteries are interesting because of their cycle life and they are made out of pretty cheap materials. They are less interesting because they have very high operating temperature requirements.
There are many more factors at play here besides density (though that is one of the most important ones if not the most important one). Longevity, cycle time, fast charging, cost and so on are all important factors too.
There are already battery chemistries that are ahead on some of these but afaik there isn't a clear winner yet for a new chemistry, but if it comes I would expect Lithium to be at least one of the ingredients, it is just too tempting a material property wise for this application.
Are we going to see a decent factor density improvement over Lithium ion anytime soon?