In my opinion, The most important factors to consider when choosing a morph that will hold its value include:
1. Considering how members of the general public (not merely hard-core ball python enthusiasts) value the morph. There are many morphs selling for very high sums today that will not stand the test of time simply because these animals eventually have to move into the general pet trade. The ball python breeding community is too small to support high prices indefinitely. Don't select a morph that you like necessarily, but figure out what most people consider striking and beautiful.
2. The inheritance pattern of the morph. Single codominant morphs can clearly be reproduced much more quickly than recessive morphs. Even more extreme are double recessive morphs. Combos will hold their value better than single morphs all else equal simply because they are more difficult to reproduce in large numbers.
3. A morph may have increased value for its combination potential and not merely the morph itself.

I think a good morph to work with is the fire ball python. Prices are low. However, fire can produce many desirable morphs. The fire itself is not particularly desirable as far as the general public is concerned. However, super fires and fire combos can be stunning. In my opinion, the super fire will have an impact on the general public (i.e. the general pet trade) on the same order as albinos and piebalds. Sure, there are other lucies... but I am convinced the super fire will prove to have the most lasting demand in the general pet trade. Some fire combos that show promise include fire bees and fireflies. Orange dream also combines well with fire.

Personally, I have purchased a large group of female fires with the express intent of producing super fires. I did the same with albinos back in 2001, and it worked out very well. Prices will always drop, but you can always sell an albino. I think the same will be seen with super fires.