1. Fast transaction times / confirmation times
2. No bloated blockchain
3. Good client that does not leak ram or require enormous ram
4. Multiple algorithms that ensure that if one breaks, another can hold it together. Ideally the algorithms should be quantum computing resistant and not follow stuff like prime numbers which might have hidden patterns / order in chaos that might be predictable.
5. Optional privacy features that are NSA-proof and Artificial-Intelligence-Analysis proof, even from AI that will be created in the next 20-30 years, which will be able to detect patterns and assemble the "puzzle" that today might not be able to be assembled.
6. Network security that is NSA-proof
7. Scarce + valuable with a good mining curve that ensures network safety and miners
8. Low inflation
9. A way to make mining, more PC-centric rather than farm centric.
10. A way to resist NSA-takeover of the network if they ever desire to develop asics for "asic-proof" coins (bitcoin is paradoxically more secure due to asics being deployed to private parties)
11. Easier transactions and friendly client - one that your grandma can use, without being afraid that her coins will be stolen or that she'll lose some, but at the same time being safe.
12. An address-shortening service (?) like tiny url for large addresses
13. An added value network within the currency network that allows people to interact through messages or voice or camera, in an encrypted/secure way, by paying the miners.
14. An AI modular interface. Currencies, up to now, are mechanistic with if/then/else. A modular currency can allow the running of it by an AI "clerk" that will process all transactions all the while ensuring all other aspects of network operation. Currently the network operation is handled by strict and "dumb" algorithms. This could change in the future by having a "mr Data" type of AI running all transactions. That AI will be neutral, but smart, in that it will be able to mitigate DOS attacks, prioritize payments, services, updates etc, protect the network from forks by sending the appropriate notifications or taking the right measures, protect the network from malicious 51% attacks and double spending - no matter the level of collusion present, make adequate adjustments in order to completely anonymize transactions or to randomize/induce chaos for preventing pattern recognition, optimize bandwidth usage or blockchain stuff, or other encrypted services running over the same network etc. It could even detect patterns of controlled nodes by one part who seeks to understand the anonymous network by controlling as many nodes as possible. To these type of nodes, the AI could send them garbage as "noise" in order to counter-them / mislead them...
* In my opinion, the next best solution to the Byzantine General problem that also eliminates the 51% threat, could be an AI solution, where a neutral "robot" (program in actuality) will be a metalayer of transaction approval - denying 51% double spending exploits. Being neutral the AI will allow everything except those things that jeopardize a coin's integrity, the network safety etc etc. We are talking about metahuman levels of intelligence here that could be possible in, say, 15-20 years.
What a list
Many of these items are good and doable even in an existing client.
Keep it rolling folks; I have no doubt that we will start seeing some of these suggestions in alt-coins soon.