Guardian Druids have some perception problems with community but there are some real ones as well.
Opening line would be that additional stack of Barkskin could solve a lot of Guardian druid problems.
Additional skill choice on Reinforced Fur( Ironfur stacks reduce magic damage by 5%(or instead increase HP by 4%), stacking up to 3 times) and talent also adds 10% to Barkskin DR like Reinforced Fur - so you would have a choice to sacrifice some armor for MR
Ursine Vigor(druid base tree) is very unpractical for Guardian druids, in between packs you would need to leave bear form, then if pack is close, stop moving(since its on gcd and you don’t want to be hit in non bear form), enter bear form instead of just Moonfire from distance while gap closing or charge into pack (20% of base armor is almost nothing in terms of DR, but HP bonus is what has the most value)
4 seconds duration is also an issue.
If you Moonfire from distance into attack 4 seconds would be over before mobs do their initial damage to bears(as those are often almost lethal) so you are “forced” to use Barkskin or SI which is “waste” of cd on every start of pull, it would be nice to have those as option mid pull or for magic damage.
One way of solving this problem would be “upon entering combat” or charging(Wild Charge) you get 20% hp and 40% base armor (lasting 8 seconds) - cd 30 secs if necessary
or New base ability - Casts one stack of Ironfur for free, cd 30-60 sec(off gcd)
Changing Ursine Vigor might not be ideal for druids in general, mostly because I am unaware with its usability in PVP as it currently stands and if that is the case, it should not be changed but different talent mentioned above should be added to Guardian druid.
Too many and too much “impactful” skill points are located on last few rows compared to other tanks.
Convoke cannot compete with Incarnation and should not be positioned as a choice on same skill point. Convoke could be moved to (5th row) in center (one below “Berserk: Ravage”). Convoke was never op for bears(considering convoke “likes” to cast wrath at enemy targets 
) but was fun and nice button to press.
Flashing Claws and Fury of Nature - 1 Point instead of 2, 15% instead of 10/20%
Scintillating Moonlight - 1 Point instead of 2, 7.5% DR instead of 10% for a single point
Thorns of Iron - Could be also moved in upper section(row 6 center, although not necessary if talents above are reduced to 1 points)
Moonless Night - Either removed or placed in upper section or added to any existing arcane damage skill point - Currently it only blocks access to Lunar Beam with very little value, its damage is very minor and very specific and for some reason it is procing from Raze(which is not a ST), Nature Vigil(druid base tree) is also procing from Raze as ST spell which is not(which is another topic).
Bear could use some mythic+ “unique” utility - It does not have any in this expansion - > Aoe interrupt Lunar beam
Lunar Beam - When casts interrupts enemy casts(once) in its range beside doing current damage and healing - Increase its cooldown to 1.5min . Currently this talent does not offer much in terms of its power in comparison to other talents in this area and is hard to get to.
With these changes you would actually have some choice in last few rows instead of always looking how many things to sacrifice and still not be able to tank properly or deal damage compared to other tanks. Choices are nice to have!
Mighty Bash(druid base tree) cannot compete with Incapacitating Roar, separating it for all druids might cause PVP disbalance but guardian could get it in its spec tree(row 3 center), in Shadowlands it was really nice thing to have and if also talented into base druid tree as well in , reduced 20% cd.
Ysera’s Gift(restoration druid tree) - Not necessary but - Beloved all druid specs talent (Restoration druid tree) was always every druid choice, not just restoration, it would have be nice to all druids and also helpful to guardian to be moved next to swiftmend(right side) (row 2) to druid class tree.
Restoration druid could have its Waking Dream on position where Ysera’s Gift on that slot or any other spell that you would find appropriate.
5-10%(depending on above changes) HP increase. Guardian druids were known as class with higher HP than most tanks and that increased HP helped them with tank buster abilities since they struggle currently with those as well.
All these changes might seem like a lot but it would still not put guardian above other tanks just more in line with them.
Some mythic+ dungeons(AV&TJS) mobs with magic damage could be slightly reworked since many tanks are at risk of being one shotted but that is another topic.