A small rework would need to be added along with 120 summoning if it were introduced; lets not repeat the way 120 slayer was done!
I feel that 120 summoning would be helpful, but I'd rather have a new skill in 2019 and then maybe 120 summoning in 2020. Having a pattern of a new skill then another 120 skill would be a good way to space out the content more.
Every requirement they raise brings Runescape closer to becoming illegal. Keep in mind that there are European laws that protect against too much content/addict-ability in video games. If Jagex were to cross the line of too much requirements to complete what they're asking of players they'd be fined a large sum of money every day for 5 years until they take the "unreasonable for most consumers" content out of game, failure to comply after the 5 year period would prevent a gaming company from selling their products in the EU altogether.
For the safety of the game itself, I'd be in support of Jagex taking a chill pill and cooling it with the 120 ideas for a little while longer. Instead, I'd prefer them to actually create and put out the content they had already promised us in the past that continuously takes a back seat to their newer ideas. They right now are like a funnel of many ideas with little actual effort put in to turning those ideas into reality. They promise us something, that gets pushed off for something new they promise us, which later gets pushed off for another new idea one of the devs had, which in turn gets pushed off when there's a new idea they come up, so on and so forth. This funnel has been happening since Dungeoneering, even to this day they're thinking about raising the level cap on more skills again while a skill created in 2010 is still technically unfinished with what content they said they were adding to finish the skill off never actually being made. At some point Jagex needs to take a step back and come to the realization that they're not a gaming company like EA or Bethesda or Ubisoft, they don't have productive devs or blank checks they can write out to deep-code developers, they don't even have a customer support team. Jagex kind of reminds me of Valve, they just have devs around just to say they do, and outside of that they just sit and make money off existing game content like TH.
Finish things first, then think about the future.Are you sure you'd like to log out? How about now?
I'll second a couple of the suggestions I've seen:
1. Instead of raising the cap, allow us to level individual familiars and customize things. Or raise the cap and tie this into leveling it from 99-120.
2. Instead of raising the cap, release a new elite skill that ties together Summoning with perhaps Prayer and Magic to create some specialized skill such as Necromancy and Demonology, as @Mutazombi suggested.