Changes to Runes, Additional Summoner's Spells
I've been playing this game since the end of Season 1, and have in general been very happy with the proactive measures Riot has taken to update and change the game. However, one particular topic has, and actively gets avoided. Runes.
These runes have been the exact same since the beginning of the game (bar some changes to numbers). In general, having them is pretty lackluster and it's just having a standard page that everyone else uses. Even if they differ a bit, chances are you won't see too much of a difference in strength between different rune pages. (That's not to say they are useless, just that one runepage is generally change much compared to a different guy with a different rune page). Part of the reason is that many of the better runes have been nerfed, thus their role in the game diminishes even further.
Of course, it's understandable that it's hard to change runes. Because unlike items or champions, changing runes affect the ENTIRE game's balance. Having any sort of update would require a long balancing work. But, this could be planned, so starting right now, so that we get an update once a new season hits.
Possible Changes: More Interactive runes: E.G, rather than having just straight MS runes, we can have ms boosts that works similar to furor boots enchantment. I'm certain the community can come up with a lot of cool ideas (post them if you have any).
Maybe rune combination bonuses. Say, as it is currently, there are some runes that are really just bad and useless. Thus the reason why people do a mixture of different types of runes. But say, someone decides to get those mana per level glyphs alongside a certain mana themed rune combination. Maybe this could unlock an extra "mana related ability" or something to make it cool to slot those useless runes in.
Of course, this will eventually cause people to once again find what the "best" rune page is, but I think that in itself is not a bad interaction so long as other combinations are just as viable.
Summoner Spells"
This is another point for consideration. I don't think there are any need to change existing summoners by much, however, perhaps it's time to add new summoner spells? Through the years, I've seen summoner spells gets knocked out one by one in accordance to irrelevance (Rally, Fortify, Promote, Revive, Surge, Clarity, etc). This is cool and all, but now we are left with even less variety.
There are many cool things that could be made. E.G, a summoner spell that stops dashes briefly (similar to Poppy W), a low cooldown summoner spell that can do a long range executing auto attack that works on minions and wards, even if they are being teleported to, etc etc.
I thought everything within the last 3 minutes, so I'm certain there are much better options. But I just want to put it out there that perhaps it's time for Runes and Summoner Spells to be updated as well.