Online play is concerned if it even has an online play mode. Multiplayer was really fun on hunters. They had good modes, Trace needs to be reworked because everybody used him due to his OP'ness but rework that, add custom character creation for online play, allow for AI controlled BOTS to be selected, a Metroid Invasion mode would be sweet similar to COD Zombies but add bosses every so often as well such as Ridley and Kraid. Voice Chat was done on MP: Hunters so it should be doable on the new one.
Leave the Single Player the main focus of Metroid like it should be but add onlilne play. Similar to how Perfect dark 64 had single player/coop story mode as the main focus but multiplayer was an added bonus that had it's own robust modes and features.
WiiMote and Nunchuck setup is a MUST. KB/M could work very well too.