My view is let the warrior be the DPS OT. SE should retool the Paladin into more of a healer/support OT.

I like the OP's idea of making flash a riot-blade combo. Here are some other ideas:

Access to more healing spells. Cure II, medica and regen are all good choices, and would make riot blade significantly more useful (by virtue of the fact that Paladins don't have a high MP cap).

Take cover off of cool-down, make it cost MP to activate and then tick MP while active.

Holy oath - Swap Str for mind, buff healing potency and nerf damage output. More healing spells would make this most useful, but even just making enhanced raise/stoneskin available in holy oath would be a start.

First aid - High potency regen, can only target other players in close proximity (like cover)

Rally - High MP cost to significantly buff one person's stats. Possibly also requiring proximity.

Rallyga - Very high MP cost to moderately buff entire party's stats