As you can note by the big ol' index above, I played a lot of RUNE last month! As someone who has both made a few Realms and played a ton, here are my opinions (not rules or anything! Just my observations) on what helps make a good realm.
On realm mechanic design:
I get that a bunch of designers have got it in their head that balance doesn't matter because you can tweak it but especially with a solo game where you're playing a pre-written thing like RUNE, a lot of people simply aren't going to do that, and will just go "this sucks" and put it down. It doesn't have to be exquisitely balanced or anything, but a baseline double check is nice. At the very least, try to play through it once like you don't know anything about how it's supposed to work - if it feels bad or too hard or annoying doing that, consider revising something.
On a similar note, double check to see if a mechanic is annoying as well as balanced. There are things I've run into that are like...this is fine on its own but it doesn't add a whole lot and it's way more cognitive load. If you're introducing a new mechanic that takes a bit of thought, consider introducing it in a "tutorial" kind of way (i.e. less threatening, gives you some idea of what to expect) before the main attraction you have in mind.
When in doubt, reduce bookkeeping, mechanics, etc. Try to think of different ways you can use one new mechanic rather than two new mechanics used in one particular way each.
On realm map/point/etc design:
Try to introduce a few different paths players can take in a Realm. An easy way is to branch things out from point 1, or introduce a few secret paths early on. Make it so that two players can have pretty different experiences.
Vary up points a bit! Have a few that are fight-heavy, a few that don't have fights, Lore, items, etc.
Put really good stuff in points you can't access without finding a secret way to that point, or behind particular fights. Make it feel earned!
Please put cards for a section near that section (like on the same spread or something). It's not strictly required but it sure makes the lives of anyone running the Realm easier.
On item design:
Keep in mind that you're not just balancing for your Realm, you're balancing for other people's Realms too. If you introduce a way stronger weapon because you have way stronger enemies then that weapon's just going to be way stronger in other people's Realms, and vice versa. This is not the worst thing in the world but it’s less interesting to have something that people either have to not use or use to exclusion, or design around.
I slightly regret setting a trend of taking away all your weapons, lol. (I'm kidding but certainly if you're going to do that, consider providing some alternatives for people to use quickly other than whatever standard two you give them - there's a reason there's 3 easy Discovers with a LOT of different kinds of equipment you can get early in Cragcliff. Locking people into a playstyle isn't going to be everyone's bag!)
The most broken players thing in RUNE is nearly always 2+ Harm at Adjacent/2 Spaces + Movement. Turns out being able to maintain range in a tactics game is really good! Who knew, right? You can mitigate this by having 1 Harm with movement, 2-3 Harm without movement, or 3 Harm + something else with negative movement.
On encounter design:
I suspect nobody really wants to be in a RUNE fight for more than like, 10 rounds. Doubly so if there's a ton of enemies with a lot of crap on them so they take forever. Plan accordingly.
Be careful with having Block being too common, that's the quickest way to make a fight last forever. If you want high Block on an enemy, reduce their Health a bit from what you'd normally make it.
One big thing you want to avoid in setting up encounters is making them "damage races". This is to say: you don't really want to have a situation where it's purely a matter of "did I roll better than them for enough rounds to win".
For all fights, use terrain. Terrain is a great way to make a fight entirely different. Introduce chokepoints, add difficult terrain to points so savvy players can maneuver enemies onto (and un-savvy players can be maneuvered onto), etc. Add new terrain too! Mechanically "normal" enemies and an interesting map is going to usually feel better than really intricate enemies and a boring map.
For standard (non-solo) fights, you can help with this by introducing several kinds of enemies. Having a few to work around, especially if they complement each other, means players have to prioritize which one is more important to defeat first.
For solo fights, you can have a moveset that does noticeably different things (attack at different ranges, for instance) to encourage adjusting tactics round to round or taking different approaches.
I'm sure I'll have more thoughts as time goes on, but these are my big takeaways.