I'm in a similar situation; I'm also a support main, but I've been deciding between adc, mid, and jungle as my secondary. So far I think I've settled on ADC as my secondary, unless the mage update really exceeds expectations and I find another champion I really enjoy enough to switch over. One thing to keep in mind is that you don't have to set your secondary role in stone; you can always be flexible, especially since you're not going to be playing it all the time. I'll list out some factors I think are important, and show you how I applied them to my selection
- Look at your playstyle and strengths (and weaknesses)
For me, I found I really like playing the map and controlling the game, which is why Support is my main (you have a lot of vision control, a lot of the champions I play have a lot of CC or just really strong CC, so you can control fights, etc etc).
Jungle also has a lot of map management, and also really controls the game. This was my first thought, however I've found that I'm only really solid on one jungler at the moment, and haven't really adjusted to playing jungle as a whole (I can clear fine, but being at the right place at the right time is something I really need to work on)
Mid has a lot of influence to roam around and manage the game. I do still enjoy mid, and as mentioned above, if I find more champions I like playing mid, I may switch over. My greatest weakness in mid lane is the incredible amount of variance; you can play vs so many different types of champions, and learning to manage all your match ups takes a lot more time than I have to dedicate to the game.
ADC has less influence throughout the map, but they're very important for taking objectives. I personally find orb-walking very relaxing, almost meditative since you get into a rhythm with your clicks and button presses. Again, since my playtime is somewhat limited, my mechanics aren't the greatest, although I can manage with attack move. My greatest struggle in ADC, as well as mid, is dealing more damage; I focus too hard on objectives and play too safe. One benefit here is that I'm used to bot-lane dynamics being a support main.
- Think about your favorite champions, ones that you excel at or can't go without playing, and if you can reasonably bring them into your intended roles
Amumu is the jungler I'm best with; I'm not good enough with other champions. Although I like playing melee champions and tanks from time to time, I'm most comfortable on ranged champions, however I'm not great a jungling with ranged champions.
Orianna is probably at least top 5 favorite champions for me. That's about it for mid-lane, though; there are plenty of other champions I enjoy playing, but not enough to really dedicate myself to mid.
A lot of my favorite champions are ADCs; Caitlyn, Jinx, MF, Corki, Tristana, Graves, etc. I'll probably focus on 2-3 of them for now, but I at least have a back-up pick if my champion is picked/banned.
- If you're going to be playing with friends, think about what they usually play and how you can compliment them
Most of my friends are Mid/Supp mains; Support is my best role so I'm not giving that up completely, but there are plenty of good mid laners I can play with
Also, it doesn't have to be just friends; even trends you notice in your matches. I really started playing more ADC because I noticed that a lot of the ADC players I got matched with were really hit-or-miss; either they were solid, or absolutely terrible (the same seemed to occur in mid lane; which is another reason I picked up mid lane). I think partially the mentality of the role attracts really arrogant players who think they're better than they are. I figured I could learn to play ADC competently and avoid having terrible games so my team has a fighting chance at winning.
- Most importantly, look at what you enjoy playing the most
Playing your favorite roles/champions means you'll stick with them and improve, also you'll have more fun
Overall it isn't terribly important unless you're really focused on only playing 2 roles and improving; you can always alternate supp/top and supp/jung, depending on what you feel like. If you want to improve, I'd limit my champion pool further, like 3-5 champs per role max, and focus particularly on 1-2 per role. If you're looking to just play for fun, do whatever you want :P