I use LLD to discover a lot. The problem with templates, LLD and external JSON API is that I want to map only single thing to item-key.

ifHCInOctet to be itemized key inbps if available
ifInOctet to be itemized inbps if HC is not available
IfSpeed to be itemized Speed if /\d{1,8}/
ifHighSpeed to be itemized to Speed if /\d{9,}/

This way I could always ask JSON API single key for same type of data without building external logic. I use Grafana to summarize multiple routers interfaces etc.

This is totally doable with today's template if I create multiple LL-discoveries. But maintaining all templates and filter rules I find troublesome. And querying same discovery data from router many times loads routers.

HC-disc: discover[IFALIAS,IFDESCR,IFSPEED,IFHIGHSPEED,IFNAME]
Speed-disc: discover[IFALIAS,IFDESCR,IFSPEED,IFHIGHSPEED,IFNAME]

With item prototype -filter I could create multiple prototypes for same key but with filter limit to only one to be created.
IFSPEED matches /\d{1,8}/ create prototype with ifspeed
IFSPEED matches /\d{9,}/ create prototype if ifhighspeed
or
IFTYPE matches /^GigabitEthernet/ create ifhighspeed, create HCOctets
IFTYPE matches /^FastEthernet/ create ifspeed, create HCOctets
IFTYPE matches /^Ethernet/ create ifspeed, create Octets

From 3.4 release notes I see some enhancements for item prototyping/low level discovery but I interpret it this way that above situation is not solved.