IMHO, if Owlcat is serious about wanting us to mod the game, the following things need to be taken care for and provided:
Exposed ruleset: that is, do not hardcode stuff. Allow us from the get-go to build new spells and feats, metamagics and classes. And eventually to go into “epic” levels - in brief, do not prevent us from going “beyond the rules”. New Mythic path could even become possible.
Actual tools + in-game modes to use them: think of what Neverwinter nights did, which allowed us to build us our own campaigns. Tools required include worldbuilding tools, including lighting & SFX control when areas, ways to introduce our own area assets (for new places to visit in our adventures).
Not to mention, a “Custom Campaign” menu could also use the same framework for further DLCs (ala NWN2).
Unity Unpacker & asset importer-explorer-exporter: explanatory. However, Unity is free to download so perhaps this could be the least problematic issue.
Ease of use for resource and asset format: again, try avoiding incomprehensible formats if possible at all. I’m personally a huge fan of OGG q10 for audio, as it is virtually lossless - but of course that’s just my preference. Also knowing how audio works, whether loop points or if similarly to Infinity Engine games, would be appreciated. As long as it’s not Oblivion-like.
In short, putting more effort in a mod-friendly framework at this stage means nowhere as many headaches later.
EDIT: add an easier way to create player/NPC soundset - including a template. More situational voice instances would be nice too.