So i'd like to start this with slayer:
At this stage there arent many decent slayer tasks to do for profit besides a few boss tasks and slayer store is out of whack (no point in buying anything with points).
Suggesting revamping shop by adding for example Sapphire boxes 5k points, Emerald 15k points and for the real grind Ruby for 40k.
Smaller tasks like Abyssal demons, nechryaels, dark beasts that type of npc add a rare drop to them something simple like legendary mboxes or a bronze mbox or something.
Even a different key with the same concept of a crystal/stone key for slayer. Could give items to help with slayer like extremely rare - nex loots ie; torva, pernix, virtus. But less rare things like regular blowpipes, vorkath's whip, chaotics, dragonfire shields, blood necks, furies, glacier boots.
Tasks in themselves: Have an option to extend a task like frost dragons example you get 7ish as a task there should be an option to extend that task by 50 kills or something roughly to gain larger bonuses at the end like 75 pts instead of 30 or something.
Add Hydra, Zulrah, Olm, Verzik to tasks available aswell maybe even the game can recognise you're maxed and can give derwens or something.
Upgraded slayer helms: Add a feature to be able to add serp helms to a slayer helm to help with venom on tasks, larger boosts for accuracy and damage with higher tier like full slayer helm (1) 10% boost, Full slayer helm (2) 12% boost. something as simple as that, have something in the slayer shop to add to a helmet at 1k slayer points to upgrade it.
Next is a daily task system:
Simple yet makes people do other things.
A task system; Mine x amount ore, fish x amount of fish etc.
Add either task points with a task point shop to have once again, boxes in them or even $10 scrolls for 400 points. things beneficial and not a useless store.
Specific amount of points for tasks, request either easy to elite difficulty so itll range from for example Fish 40 rocktail as an easy to fish 3.5k rocktail as an elite type of thing. range from 1 point for easy to 15 for elite. can do as many tasks a day if you want so if you have time to grind and dont feel like bossing you can. also makes life nicer and more appealable to skillers.
Next in line:
Add a price to sell or alch item that theres an abundance of like blowpipes, dragon kites, glacor boots, pvp armours, etc. like blowpipes on the market are 300-400m. maybe add a alch value of 200m so people that have an abundance of $ would just alch them. Dkites are 200-250 so alch maybe 100m? itll drop a fair few of them out of the game and make the grind a bit better rather than selling votes and getting a good enough setup to skip all early game stuff. if there arent many in game that arent being used people wont sell therefore people are forced to do bosses etc which will make the game in more enjoyable in the long run.