What if they let you break the haste cap if you had a shield on? Pretty much the same idea, but I don't want to put words in your mouth, if that wasn't your implication.
I think we are aware a shield doesn't make up for an offhand in any practical situation, much less the one where a RDM actually has Fencer.
I like the potential on the shield bashing. The implications could go either way on it. If you make it so you have to be blocking to get attack procs then you would be pushing a RDMs roll as a tank. If you make the shield bash procs independent of actually blocking, then a RDM doesn't have to be tanking to get the benefit.
Then you have to decide if the shield bashing is about doing DPS/TPing or if it is about utility, aka stunning. I guess it could be both. If this only applies to jobs with Shield Mastery, then I would expect that Bucklers and smaller shields would hit more frequently for better DPS for RDMs and larger shields would hit less frequently, but for significantly more damage for enmity gain for PLDs. You could even give higher ranks of Shield Mastery and heavier shields higher chance to stun to give PLDs some extra utility.
As for the possibility of a passive Cover, I would love it. I throw myself in front of anything that is being attacked that is squishier than me with curing anyways. In a lot of situations, Cure IV doesn't do a lot to actually keep people alive by virtue of HP cured, but because spamming it grabs hate more than a single Cure V or VI. Some days I wish we hadn't been given Tranquil Heart, but other days I'm more than fine with it.