My answers, for what they are worth:
Complexity: Go for simple at first but have an eye toward expansion. Try to make it so that you always have a playable game. This will keep you interested, so they say.
Character Building: Anything can work well. A straight class system, like Cyborg, Pilot, Ninja, Quarterback, etc...those can be fun. Or you can go with races you create with sweet names like Nalwick, XXyyZiCks, etc...or you can do a fantasy conversion straight across, with Humans, Belters (elves), Heavy Worlders (dwarces) and Far Siders (halflings) and the classes be Solder (fighter), Medic (priest), Scoundrel (rogue), Psychic (mage)...my personal preference is to toss character building all together and just jump into the game like Powder and Brogue.
Movement - No preference, but many people HATE 4 way.
Wish List - No leveling. Advancement in game is through gear collection.
RESOURCES - Check out the old sci fi RPG STAR FRONTIERS. There are several books all freely available online. Some good ideas in there. Also ALTERNITY was pretty good for ideas.
Hub Town - I like persistent dungeons and no town. No quests. Just the main goal of kill the baddy or retrieve the epic item of awesome.
Unlockables - Fuck that. I have very strong opinions on that shit. Fuck it. All of it. Don't nerf the game out of the box. Please. :-)
Game Length - 30 minutes. You can beat the game quickly if you are good enough, but you'll NEVER see all the content in one game, or 5 games for that matter. Binding of Isaac did this well.
EDIT: What's most important, probably, is that you make the game YOU want to play.