Here is a simple thing which i first saw on herecy and i think it will work here. Its quite simple take the quote "I would ...... if ......" putting in a word or 2 into the space. for example .
I would run an army entire Far Harad army if the entire army wasn't metal models

It does give alot of thought for example would you use more cave trolls if they were in a plastic kit. Would you go dol gudor if GW give use the Dark Hourns and Werewolfs that the promiced us years ago.

I would collect a Khandish army if they had plastics. (Likewise with Far Harad though I'm considering a small warband of them anyway).

And Cave Trolls are in plastic. It's called MoM and deals through ebay

Similarly:

I would collect Dunlendings if they had plastics.

I would collect Black Numenoreans if they had plastics.

I would collect men Carn Dum if they had any models at all.

I would collect more fiefdoms if they were plastic/had more models available.

A lot of these armies I could do with non-GW minis, but I'm also a little hesitant to get alternative minis in the unlikely event that GW does eventually release them. For example, I have a large number of Rohirrim which I've been sitting on for a looooong time, planning to convert them into men of Esgaroth (this was long before The Hobbit movies were confirmed) - now I kind of regret that I've got all these guys because we're getting those movies and, by extension, probably new minis for them. Eh, we'll see how it goes.

On the other hand, the forces I mentioned are maybe less likely to get a plastic set in the near future, if at all, since they'll never get screen time.

it is always a problem. I have the same problem as i play at my local GW alot of the time so i can't use non-GW stuff there even if it's not on the store floor. Sucks sometimes

How many would you collect though? Given current pricing, I'm assuming even if GW brought out all the armies you wanted in plastic, you'd still only probably collect one, maybe two just because that's what you could afford. There's a fair few armies already out in plastic.

Personally, I would definitely buy Khand as I can do so much with them paintwise. I'm actually tempted to buy them metal (and convert chariots) but the price is prohibitive.

I tend towards the smaller end when it comes to battle sizes, so I like having a handful of everything rather than full sized armies.

Of those I listed, I'd readily get a box of each and a command set, and then if there was an army that I was especially interested in I might build it into a full-sized army later. As is, I have good-sized Mirkwood Elf, Moria Goblin, and Mordor armies, but at least some troops of almost everything that's available in plastic, and a bunch of hobbits.

i know what you mean. I have about every army which a fiar size of each to get to atleast 300-500 pts min. However becuase of that i can';t play WOTR mush (even if i wanted too) as i would have to load up on a million allies (seriosly my only armys to be able to play are Gondor and Goblins and even then i have to load up on epic heros and monsters)

I don't know anyone in my area that plays WotR, so I've never given it a shot, but if I did I think the only army I'd be able to pull off would probably be Mordor - and I would definitely need to fill those points in with some Nazgul (and probably Fallen Realms allies, too).