There are a lot of things SSG could do to fix the broken blue warden tank, but it would be a lot of work. Based on Severlin's comments on class balance it seems obvious that SSG doesn't have the resources to devote to fixing Warden tanks any time in the foreseeable future.
Given the reality of the limited resources that SSG has, I'd like to suggest a simple fix that won't make Warden tanks competitive, but it will be a big step in the right direction, just increase blue warden self heals by a factor of 10.
I just did a quick test on a dummy by keeping 100% uptime on tier 1, tier 2, tier 4, and conviction self heals as well as celebration of skill for the inc heal buff. Keep in mind, this rotation is not realistic in real fights, it leaves literally no time for taunts, damage, or mitigation gambits. Doing nothing but self heals and celebration of skill yields about 17k HPS at its peak. If you increase this by 10x, a Warden could break 100k HPS on themselves while executing other necessary gambits during real fights.
Warden tanks still won't have in combat rezzes, group buffs, a way to protect their fellowship, in combat runspeed boosts, a working panic button (i.e. last stand), or a number of other skills that captains have which would keep them far and above better tanks than Wardens. Still, you would be throwing us a bone and improving this utterly broken spec with literally 0 work/resources other than an engineer multiplying the skills I noted above by 10 and limiting those #'s to blue line only.
Come on SSG, do something for non cappy tanks, please.