I've never had a problem with skill "bloat" on the guardian (N.B. I don't play red), even when I was totally new to it. It's a pretty straightforward system, after all: something happens, play a skill, play next skill, if you don't, you lose the opportunity. I think it's actually quite fun, and certainly not something that you'd improve upon if you changed it as described. Forking response skill paths are really interesting.
It is true that the game is terrible at introducing itself, but that's an issue with tutorials and available information in-game, and has nothing to do with the guardian. For example, class quests don't provide any guidance or opportunity to practice, and the way you unlock skills (i.e. they automatically appear on your bar) doesn't especially encourage people to read them, either. (By contrast, buying them from a trainer makes you read the skill, right? You want to know what you're buying. And you could do much more to "sell" a new skill when it's first unlocked.)
Regarding the guardian, the following things come to mind:
- I like my guardian because I have a clear and direct tool for many problems (e.g. interrupts, corruptions, debuff clear, good variety of tactical/emergency skills, nice set of taunts) as well as a rotation that's relevant and not always the same (e.g. block/parry responses, Shield-Smash is neato for easier [read: stunnable] stuff, Redirect + War-Chant is neato for harder stuff). Guardians feel solid and capable when tanking. When starting out, you can do a lot with a few skills, and then you can add more skills as you become more skilled.
- In some fights, block/parry responses don't show up that much, which makes the class a bit boring to play.
- There are and you need to use a lot of animation cuts. I don't mind this, actually, but it's probably not good for accessibility.
- Yellow line used to be so cool, with Radiated marks and AoO skills proccing multiple Flash of Light hits per target. I miss that. The non-scaling of Flash of Light was/is great for tanks, too. That is, if you didn't have mastery (say because you forgot to bring your DPS swappies, not that that ever happened to me), Flash of Light could rescue your DPS--the damage wasn't actually that high, of course, but significant on a tank build. And if you were tanking, you'd generate a good amount of aggro with Flash of Light, but you'd really want to draw in additional mobs (at least 4 at the time) to Radiate marks to. I like that: scaling benefits for engaging multiple enemies (scaling even more than regular AoEs already do, that is). I wouldn't mind seeing Radiate work on bleeds again, either.
- Aggro management is (almost) all taunts. This is a problem with all tank classes, but still worth noting.
- Defence is (almost) all mitigations. Again, a problem in general. As you note, SSG has painted itself in a corner (I'd say: ####ed up) with mitigations. The stat system is ####ed up, too. People used to have BPE swappies and resist swappies and such. You can forget about that these days.
- Swapping between two-handed and sword-and-board isn't something you can do in combat anymore, and even if you could, you don't have the stats to be a "temporary tank" from redline or a "temporary DPS" from blue or yellow. I consider that a loss; swapping roles in combat is great, and tying roles to a particular kind of weapon makes a lot of sense (i.e. "drop the shield and grab your BIG sword for BIG damage" is cool and intuitive). Reintroducing weapon swapping probably isn't going to happen, so maybe separating weapon type and role is the thing. Although that'd also be a loss. Making guardians sword-and-board only (either formally or practically) would be bad, as well. There should be lots of ways to run a guard, not One True Setup. (This is also a general problem. I smell a pattern emerging...)
- Why do tanks need so little finesse? Too many of the important skills can't miss (interrupts, taunts). Finesse should be a tank stat par excellance, as it improves the consistency of your make-or-break skill plays, like taunts. But you end up slotting all vitality/tmit. Perhaps the threat system is too heavily taunt-based to make taunts unreliable enough to require finesse, hmm? Itself an example of bad design, of course.
The emerging pattern is that successive updates have replaced complexity/depth with bloat. Different pieces of equipment have been replaced with "just stack essences". Different defences have been replaced with "just cap mits, bloat morale". Weapon swapping is gone. Avoidable interrupts are gone. Since trait trees, guardians have like 40 tanking traits/specialization benefits, but only about ten are relevant: Pledge, War-Chant, Fortification, Take to Heart, Redirect, Bring on the Pain, Juggernaut, Smashing Stab, Shield-Smash, Numbed Senses, Break Ranks. And most of these you're just going to have anyway, so all the trait choices you're making are about secondary or tertiary traits (plus specialization, which is always blue, so yeah).
Overall, I'd encourage you to keep the guardian as a moderately complex class, and update the stupid class selection page (in character creation) listing it as "basic" class. No tank class is "basic". They all require good situational awareness and melee positioning, and tanking is almost always a more difficult role in a group than DPSing. In part, this is because DPSers have had a lot of their support abilities stripped out, so they tend to have one job: hit the target assist. Again, red guardians (or red wardens, nevermind champs--burgs can do it best, I guess) are very unlikely to be able to Challenge and take over some adds for even 10-15 seconds, and I don't think any DPS runs, say, a CC rotation while DPSing. Now, if a red guardian could legitimately take some pressure off the main tank because it's a "tank" DPS, that'd be really cool.
So yeah: focus on encouraging multiple builds and cut out bull#### traits.
Andhilin, Ifeyina, Iondhilin, wardens of Gondolin -- Till shade is gone, till water is gone, into the Shadow with teeth bared, screaming defiance with the last breath, to spit into Sightblinder's eye on the Last Day.