That's a good question, and deserves an equally good answer.
First of all, this game is to me, no more than Diablo 2.5 under a different flagship. I'd like that to change. It can't really compete with Diablo III, although it probably does hold a candle to it (no pun intended). What I would like to see it do is outshine its spiritual predecessor.
I'll try to put it this way: I love this game. I want to take it to the next level, and do so in a way that I find as pleasing as a would-be game developer does on his own first title. While this is the biggest project I've ever taken on as this sort, I do intend to take FULL ADVANTAGE OF the Editor... because GUTS is essentially an OGRE-Engine Clone. This is my basic answer; I'm working on VIP in order to take full advantage of GUTS. But, that isn't everything. My VISION is grand, a larger picture than the game is, itself, currently. But, that isn't easy, and OGRE isn't an Easy Engine to TOP. Guts takes awhile, it's tricky, and handles rather clunky compared to what it could be (bearing in mind - it's only 32-bit).
So, that leaves just one thing I'm trying to do with VIP: make an Expansion worthy User-produced Modification. That is my aim and it is literally a long shot. I may give up, I may quit, I may not find the talent I need to work with me or the help I need to see my own mod integrate multiple mods within its structural system... but I will try, and try my best.
Short answer: to test whether I am better than the tools I have. I've always wanted to be a game developer. I am 36 years-old and have never started a real project before, only theorized, planned to, and/or stated it. Now, I have the time, resources, and a great co-developer (my girlfriend).
This mod will take Torchlight back to its Diablo clone roots, and make the ultimate, realistic, creative ARPG. I've even pondered actually cutting the project from the Steam Workshop and drawing up my own game. I believe in Runic Games, though, and their community believes in me... therefore, they must believe in me. So, finally, I will make the mod they would be, if they weren't burnt out, too busy to expand TL2, or simply working hard on other games (they are a multi-title company now after all, NOT JUST TORCHLIGHT!

I want to carry the torch as a leader of mod makers if I may. Short and sweet.