Themes
I think there are a couple of ways of looking at it. Cecil from FF4, as the archtypical PLD, is predominantly a physical attacker with some minor white magic for utility. This might be partially a localisation issue (the JP version is Knight). That being said, most of his basic attacks do carry a magical 'feel' to them, as the weapons that he wields apply 'holy' styled animation effects.

There definitely are more holy-based sword attacks out there, however. Shock, used by General Leo (FF6)/Beatrix (FF9) is a bit of an obscure example, but it's a good one. FFT's Agrias likewise plays a bit closer to what NA/EU players might expect from a holy knight, by channelling holy spells through her sword. I feel that Stormblood's introduction of Requiescat and Holy Spirit represents a move to better represent the latter style in PLD's gameplay.

Gauges
While I can understand the desire for a more utilised gauge, adding dps options to the Oath gauge will devalue Sheltron. This is already a bit of a problem with Intervention, in that it's dps neutral, while Sheltron is a dps gain. Any additions to the gauge system will need to provide a certain amount of equivalency with Sheltron.

I do think that they should make the resource gain consistent across both stances, though.

Abilities
I think Flash is a carry-over from FFXI. I think most people dislike the no dps element of it. One possibility could be to implement Shock as an upgrade to Flash in an expansion, and turn it into an AoE threat move which also deals holy damage.

As far as Holy Spirit is concerned, I think it creates a nice bit of aesthetic variation by being a spell. On a related note, one thing that I'd like to see on either PLD or WHM is the white magic spell Float. I'd like to see it implemented as a spell that removes movement restrictions while casting. It might be nice to activate Float, create a pair of wings, and then zip around the battlefield while throwing out Holy Spirits without cast interruption.

Either way, I do think that PLD needs some sort of movement skill or gap closer. If they do this, they could probably move Tempered Will into the role action category, as most roles have at least one action that fits this category on a significantly lower recast.