I am generally into old school gaming and innovative games and/or sandboxes. And as a timeline I either like medieval fantasy or futuristic
A very promising game is Project Gorgon. Has a lot exploration and an Asherons Call/DAoC feel although its not much PVP based, at least at this point.Also has innovations like some bosses can curse you and give you their form. Like Cow, Spider, Werewolf form and each coming with their own skillset
Another good one is Albion Online. While graphically it feels somewhat like improved runescape it has all you would expect from a sandbox and the devs saying they want to build an economy like EVE online
Perpetuum Online is a buy-to-play sandbox with Mechs. Like most indie games it has at times very low population but it has promising features and is being updated 3-5 times per year. Overview and other things remind EVE
Mortal Online. A first person view unreal engine sandbox that draws inspiration from the hardcore era of Ultima Online. As first person, obviously you see your hands from first person view like an FPS. It has deep crafting and all you expect from a sandbox but the biggest draw might be to hardcore pvpers. The game has little to none protection right away and if you stick to populated areas as a newbie you most probably will get griefed. Best course of action is find a guild first. Like EVE most guilds require use of TS3
Star Sonata is a spaceship game heavy on sandbox features but not fully a sandbox. It is 2.5d and draws a lot inspiration from star control game series. You can build your own starbases, craft things etc.
Ashen Empires is a very old game with a feel between UO and Linkrealms. Due to its age it doesnt have a big fanbase right now but its safely a great gameplay experience and community is friendly. Of course updates dont come often for such old games and typically like in all old games you will whines about it on forums from players who play over 5 years and done all
Ultima Online Forever. Its an ultima online shard that seems not having technical issues often and is actually being developed too. Last time it had a big community for such an old game and finding friends and guilds seemed fairly easy. The weird thing is I tried the original servers too around same time(8+ months ago) and UOF server seemed more populated. It may have to do however with the fact the original developer has the players split into many servers for such an old title.
Now I got a lot more ideas like Anarchy Online, Everquest, EVE but I think these are enough already. Moreover we dont know what will happen. There is a chance mythyn will continue development and keep servers online, time will show. By the way my post is marked as spam if I place links for the games