That's opening a big can of worms, LOL.
My 2 cents:
If it's truly for investment, I'd advice you against knives and to look into something more...conventional.
There are way too many variables to go into much detail, but you need to know the market and take a guess at what patterns and handle materials will be in demand X number of years from now and plan accordingly.
For example, in the 80's the "American tanto" style was hot, but it's fallen somewhat out of favor today. So, someone who bought a bunch of customs back then in this style might be a little disappointed if they tried to unload their collection for huge profits in today's market.
Besides style, an even bigger dynamic is the knife's maker. A genuine Bob Loveless knife in poor condition and relatively unpopular handle finish, will still fetch more than a popular type made by Joe Unknown.
All that said, if you're collecting because you enjoy knives, not as an actual "investment," then just buy what you like and don't worry about what or who is popular.