Panels sounds cool and all but why would I want to spend hours in line with disgusting nerds who I can only imagine are the most cringe worth meme spouting idiots on the planet when I can sit in the comfort of my own home with some decent food and drink and stream it in HD on a 40 inch TV?
I get most panels don't stream but the ones I care about usually do.
I'm just guessing that BL has a more active community that would be easier to get things accomplished within.
Nope, Borderlands modding is not even a thing unless you are talking about using willow tree to make new weapons from exisiting in game parts.
If you genuinely were interested in doing something like creating an entire sandbox for competetive play then you have a few options.
Halo Custom Edition Open Sauce
You get that Halo Gameplay feel but its going to be limited in how you can implement things and I think hard coded tag limits could hinder your vision unless
Source Engine
Its got lots of wiggle roam for freedom but the developer tools are not the best they could be and you are going to need to learn how to code at a above average level to get things running
Unreal Engine
Its got a good toolset from what I hear but it means you are going to need to learn UnrealScript.
And obviously you have things like CryEngine and Unity.
Modding Halo CE OS would be the best bet and the easiest.
Halomaps.org already has a decent collection of existing weapons tags from the Halo games which you could modify. Outside of that people have made loadouts (im not sure how well the work) Obviously you would have to model your own unique weapons but if you use the already existing tags you wont have to model everything.
Scripting some of the more ... obscure weapons would be difficult