Aside from cosmetic things, like most of the compendiums on DAI modder, are things like SweetFX presets, ENBs, and reshaders available on nexus, if you find you want changes to the lighting/color vibrancy of the game.
There's also a mod to change the default text font in the game to gothic, resembling DA:O in that sense. I highly recommend it. Its a seemingly small thing, but it really changed the feel of the game for me, personally.
(There's also a font mod that changes it all to a less fancy font if you need to increase ease of reading.)
And there is the conversational camera zoom mod. Compared to previous DA titles, Inquisition has fewer conversation cutscenes. Instead you talk to landscape npcs and the camera just zooms over your shoulder, while still staying relatively far away from the NPC you are talking to. The camera mod I mentioned zooms the camera in closer over your shoulder in landscape npc conversations so it gives a feel closer to what it was like to talk to *every* npc in the previous two games. I personally highly recommend it, even on a first playthrough.
Finally, there is a mod that allows you to run around the game world in first person. The game wasn't made with first person view in mind, but the modder has done an admirable job implementing it, imo. I haven't actually gotten around to using this, yet, but it looks cool.
Just be prepared for your mods to possibly break next time there is a game update. I'm personally waiting till the DLC/update cycle is complete before I do a fully modded playthrough.