If I've missed some option that allows me to do this, I apologize, but here is my suggestion:

With selecting a game to spectate, I would like the ability to search for a match that has a certain champion in it.
I bought Malzahar and am using him as my main champion, it would be nice to search for matches to spectate where people are using that character, so I could get ideas on how to play better(there are probably other reasons for doing so, this is just what interests me).

Probably been said a million times (don't really feel like sorting through 131 pages to make sure), but would love to see an ingame chat between spectators (similar to WC3 observer mode)

would very much like to customise my own hotkeys for spectator mode. currently team vision is set to f1 f2 and f3-keys which are locked to screen brightness for my computer.

In Game HUD quality/cleanliness is kinda **** after watching several spectate games. I think that spectator mode is so clean and easy to look at whereas the current in game HUD is fat and takes up alot of unnecessary space. I was wondering if anyone intends to update the in game HUD cus frankly im sick of dealing with HUD installers and RAF Manager. If Anyone at riot reads this i hope you think about updating the in game UI aswell

A ability range interface set as a optional setting:

The abilities of the selected champion is revealed. Abilities with range or aura fall under this option.

How this would assist the community: Ever wonder if that Sona is covering everyone in her Aegis aura? Now you can.

Abilties could be colour coded, or numbered or alphabetized such as QWER/1234

I dont know if you would even think about implementing it even in normal games or only for spectator mode as an option for players if they want to have that added control although it risks making the game too cluttered so making it a option should help.

+ or - would be helpful form the community if you think its a bad or good idea.

I would love to see the primary HUD put subtle highlights on any champions that are on screen. It would make it easy to check items and gold for the players that you are currently spectating. It's hard to describe what I'm talking about, so here's a mock up instead.

Is spectating Ranked games disabled as of this patch?

I like the ability to view ranked games, but... it'd be cool to allow spectating games of any summoner if they're in a game when you view their profile. Another suggestion would be to let people spectate random games within a specified ELO range, not just viewing those high-ranked blokes.

Don't know if it's been told or anything, but a message saying that the "actual" game is over would be nice, I've watched a couple of my friends games, while waiting for them and they hopped back into another queue while they thought I was spectating elsewhere.

I also vote for an in-game chat feature, Between people on your friends list and/or other spectators. You can also make it so you cannot chat with individuals in the game you are spectating, even though there is a 3 minute delay you can still tell them patterns or info about the game that they are in.

Events (deaths / multi-kills / drag / baron, etc.) should be identified with a marker on the timeline so you can rewind to specific scenes.

Death recap's for slain champs should be able to be seen while that champ is dead somehow.

Champ stats should be vieweable like you can see your own stats in-game.

Full-replay should be available when you join to spectate.

In-game players should be able to vote to allow a spectator to join the game in place of a disconnected player if they feel they aren't coming back. Require a timer before the vote to allow the DC'd person to get back within a reasonable time. If it's a ranked game, ELO should help with the selection process, otherwise random wins, and spectator chosen obviously gets the "accept/decline" option... declining restarts the selection. Ranked teams subs get priorities, should they be spectating.