So you've made a successful mod. Congratulations! Perhaps you've heard of the incredible success of mod-to-indie transformations like PUBG, Team Fortress 2, DOTA, The Stanley Parable, and Dear Esther, and you're thinking about transforming your mod into a stand-alone indie game too.
I'm going to offer some advice, based on my recent experience. I made a story-driven mod called The Forgotten City, for a popular RPG I'm not allowed to mention, and it's been downloaded by 1.9 million people, won a national Writers' Guild award, and received broad critical acclaim. Kotaku even called it "ambitious enough to be its own game". So, I'm re-imagining it as a stand-alone game, to be released in 2019.
Re-imagining your mod as a stand-alone game is a massive undertaking, and you're going to want to do some exhaustive preparation beforehand:
1. Get legal advice
Chances are, you agreed to some kind of End User License Agreement (EULA) with the developer of the game you were modding. You probably didn't even read it. Unfortunately, it may have terms which restrict your ability to re-use or promote the intellectual property you created. It's going to depend on the terms of the EULA so there's no one-size-fits-all approach here. But if you don't get legal advice, you may get a long way down the road to making your game, only to be the subject of legal action from that other developer.
The law around intellectual property ownership of mods is shrouded in mystery, so the best approach is to reach a clear written agreement with the other developer's legal team, which removes most of the uncertainty. That's what I did, with the help of a specialist video game lawyer. I recommend trying something similar.
2. Ask yourself why your mod's fanbase would pay for an experience they've already had for free
This is a tough question, but a necessary one. To market your game, you'll most likely be relying on support from the fanbase of your mod, at least in the beginning. Ask yourself: ask yourself why someone who enjoyed your mod would pay $20 for an experience similar to one they've already had? In other words, what improvements are you going to make to your game that weren't present in the mod, and how are you going to communicate them to your fanbase? For The Forgotten City, I'm adding a new setting, new twists and endings, upgraded visuals, a professional orchestral score and voice acting, mocap animations, among other things.
3. Talk to your fanbase about what they want
It's incredibly difficult to anticipate what gamers want. You can speculate all you want, but the best way to figure it out is by simply asking them. Consider running a survey, via social media or SurveyMonkey, to gauge interest in a stand-alone version of your mod. What do the numbers say? Which new features are they most excited about?
4. Estimate how much it's going to cost to make a prototype
Making an indie game is expensive. You're going to need funding from somewhere (discussed below), but before anyone will even consider funding your game, you'll need to build a prototype. You might think "Well, I have this mod...". Sorry, your mod isn't going to cut it. There's a massive amount of work involved in re-building a mod as a stand-alone game, particularly if you're using a different engine, and every potential investor knows it. They'll need to see a stand-alone prototype, which you'll need to put together at your own expense, before you seek funding.
a. Prototype Expenses
Here are the things you'll need to budget for:
There's no one-size-fits-all equation here, but as a ballpark figure I'd recommend setting aside at least USD $30,000 if you need to hire specialist help. It'll be cheaper if you do everything yourself, or partner with someone who's giving up their time for free, but having the support of an experienced professional is a huge advantage.
b. Opportunity Cost
I'd recommend setting aside a minimum of six months to make your prototype if you're working full time, or a year part-time. Remember that if you quit your job to do this, and your annual income was previously about $50,000 a year, then your opportunity cost is $25,000, in addition to your direct expenses.
5. Figure out how you're going to pay for development of the rest of your game
Unless you're wealthy enough to drop a few hundred thousand dollars on your game business, you're going to have to secure funding. There are four main sources of funding:
Taking the leap
If you're confident you can handle the above challenges, then you may be ready to take the leap and begin turning your mod into a stand-alone game. Indie Dev is risky and expensive, but it can be a wonderfully satisfying, and lucrative, business. Good luck!