Megaman Online or (or simply Megamen), or maybe Rock On overseas.
Third person action Megaman online game, where everyone has their own customized personal blue bomber (or red, yellow, whatever) and ability.
Earth is one big open world MMO with missions [...]
You had me up until here. Legends x Monster Hunter is what I want - but open-world, quest-based, generic MMO design is not.
I want similar structure to a normal Legends game, totally completeable offline if you really want to, but with online (and ad-hoc) hunting in groups. Multiple islands the size of Kattleox from Legends 1 - around 3 or 4 would be good, but since we're dreaming, I'll say 8 to follow Megaman tradition. On top of that, have several mini-islands, sized like those from Legends 2, which basically amount to just one town or one mini-dungeon. 6 reasonably, or like 16 since we're dreaming. Areas would be zoned, not open. In fact, I think it'd be best to also change up the dungeon design a bit to suit the MH group play: no more long tiny corridors, smaller zones, stuff like that.
You'd start out on one of the mini-islands, which would have a small town and a small dungeon. You're a customizable Digger of ambiguous backstory whose airship has run out of juice. You take on the mini-dungeon on the island, jumping and shooting and pressing switches Legends-style, acquire a new refractor, and use it to repair your ship, which opens up the world map, like Legends 2.
From there on, every area is accessible in any order, though they're not all exactly the same difficulty. One area you can now access is the Digger's Guild island, which is the hub from which you go online and join groups with other players.
For weapons, like Legends, you have a buster always on your left hand, and a special weapon on your right. The buster can be upgraded with equipped accessories, just like Legends, but you can also pay some amount of zenny to fuse parts into you, so you get a permanent bonus without using an equip slot. The fused bonuses would probably have to be lower than the bonuses for equipped parts, and fusing would have to be quite expensive, for balance reasons. To get new parts, you'd have to sell junk dropped by enemies to a junk shop in certain amounts, like Etrian Odyssey.
You can choose one special weapon at the beginning, and the other choices can be made from junk that enemies drop. You'd also get a unique one - or the parts to make it, I guess - from the bosses of each of the large islands. The special weapons are more like MH weapons than Legends ones; they're all relatively balanced in power, with their effectiveness depending on what you're fighting. They would all play fairly significantly differently: the beam sword lacks range, the rocket launcher is super slow, the machine gun is usable on the move but has very low damage, etc. There'd be no Shining Laser or Active Buster to break the game in half with. Maybe as a postgame reward that can't be used online, or something.
Monster Hunter's item system would be good to have, too. Instead of potions, you'd have energy canteens, but upgrades to their size would have to be more limited than Legends, so you had incentive to carry more than one. The other Legends items, like the vending machine drinks, hyper cartridge, shiled repair, chameleon net, flame and light barriers, and so on would all be used this way too, as well as some MH items like expendable bombs of various kinds, traps, you know.
This has become a much larger post than I itended, but basically, I just want to play a new Legends...