Characters in HxH can use more than one group- they just aren't as good in them.
Erza would be materialization though, not manipulation.
Huh, weird. Based on her personality and style, I'd always considered Erza a reinforcement nen-type who makes use of some materialization/manipulation techniques with very specific conditions. You'll note that her weapons always have a purpose. Some reinforce her attack power, others increase her defense against particular types of attacks, etc... Overall, she's a physical brawler who gets extra-boosted from using special abilities--which are the special benefits provided by otherwise normal weapons and armor (normal stuff probably isn't that hard to materialize, all things considered).
One of the awesome things about the nen system is that it truly allows for unique abilities without being ridiculous. After all, abilities are custom matched to the user's personality and the mangaka doesn't stick to generic, broad things like "controls fire" or "controls magnetism". Abilities are specific. It also allows for a much fairer power system. After all, abilities are so conditional that a person rarely wins by being outright stronger inherently, which tends to ruin the power hierarchy of manga as the protagonists must accelerate at ridiculous rates in order to overpower all opponents.
Oh yeah, and lastly, Hunter x Hunter allows for out-of-combat powers. How awesome is that?? I can't think of almost any other series where there are non-combat-heavy superpowers.
About designing our own nen...I think I'd probably be a Materialization or Manipulation type, based on what little I know of myself. Maybe somewhere in-between the two like 90% aptitude in each, since they're close together and I'd guess people rarely fall perfectly on one spot on the pentagram (or was it a hexagram?) scale thing.
As for my ability, I'd like to try something a little...interesting using manipulation (I think, a little vague on exactly what it does) and materialization. I'd like to go for the power to manifest a contact lens which allows me to see exactly what my eye would normally see, only a pre-set amount of time in the past/future. Maybe just the future if the past would be too powerful and unobtainable. Upon putting the contact lens in, I would say a specific time (e.g. 2 hours forward) and from then on until I took it out (or I ran out of nen), I would see exactly what I would have seen out of that eye two hours in the future. If I wanted to fight, I would obviously set it to something more like .2 seconds in the future. Here are the conditions to make it more accessible.
1) I must actually put the contact lens in. No just materializing it on my eyeball or something like that.
2) In order to change the time, I must take the lens out, dematerialize it, materialize another, and put it back in.
3) This is one penalty I haven't fully decided on as it's very tricky, but it might go something like this. If I react in such a way that I change the future and will no longer see the vision that I already saw, the lens dematerializes and I can't use it again for a day. Or maybe for 10x the time difference, haven't decided exactly on that penalty. This means that if I see an attack coming straight towards my face 2 seconds from now and decide to dodge, I better dodge in such a way that I still see that attack coming in two seconds later. So if I see 8 hours later the winning lottery numbers displayed online and the name of the winner next to them, I better go out, buy that lottery number, then reprogram my computer so that it changes my name (the winning name) to the other name on just my computer when I check it later. That way the context may be totally different, but I see the same stuff.
Here are the limitations that make it less powerful and costly than it might seem.
1) I'm badly nearsighted and normally wear contacts or glasses. That means that in order to use this, I would either have to take out the one already there and put a contact in or just always walk around almost blind in one eye.
2) Requires preparation and at least a little time--even at Killua speeds, it would take part of a second to put in a lens, longer while moving.
3) Whole have-to-see-it thing. So if I see someone dying or something, I have 3 choices. A) Rig up a trick so it looks like they're dying from my angle. B) Lose my ability for some time. C) Do nothing
3) Can only see what will be within that eye's field of vision later--no looking around, pausing, etc... Which means I would probably always be looking everywhere like a twitchy crazy person to make sure that I will have seen everything earlier.