(1) I might be alone on this, but thieving on a prime is a pain for a pretty simple reason... Ham members from 15 to 38 for master farmers, that in itself isn't that much of a chore, although a 25-30 thieving requirement NPC would appreciated, but it's the leap from guard (40) to Paladin (70). That's HUGE, for Primes. That's a bigger leap than Agility being 35 > 52 and Agility even has A MASSIVE experience advantage over thieving for those levels.
(2) Updating the guides on clickable/trainable skills. It's kind of weird/daft to have to ask people to google OSRS potion lists, or ingredient lists or even silly things like, updating that you can't cook sharks @ 76 cooking, because players that have skipped playing Runescape or just haven't played in a very long time, get confused by that weird typo.
(3) 'Trial mode' I'd LOVE, an instance where I can attempt a boss that I've never done, without having to spend 6 hours worrying whether I haven't memorised a rotation or pattern and instantly losing a Prime HC. Hell, I'd even love if you had to pay GP to do it. 1m per trial and ensure that it's a safe death but has no rewards, what so ever. Just to learn the rotation a bit. Maybe even limit the amount of times you can do it, or increase the GP per trial.
(4) Add new farming seeds and ways to get farming seeds in more bulk (mainly lower tier seeds). Considering that skilling tasks exist, a way to better farm for seeds would be a god send, at this point; and I truly love farming but it's so repetitive to use the same few (sparse) seeds and herbs. It'd just add more content to those of us who go for max and care about life after max (200ms).
(5) Not sure if this would be do-able, but i'd be curious to know & would LOVE for it to become a thing. ZMI. Random runes, slightly extra experience with the chance of dying. Runecrafting isn't boring due to time spent, it's boring due to repetitiveness.