The 'Whats Old Is New' game system holds alot of promise. From the Kickstarter campaign:

"These are crunchy games! You can spend hours tinkering with them, or just throw yourself straight in! Play itself is designed to be fast, but both games allow you to spend as long or little as you like on the "back end". So if you enjoy optimizing characters, or building starships, or designing planets, spells, magic items, settings, races, and careers, you can - and it all funnels down into easy rules in-game."

The author states that these products are the result of years of tinkering and development.

The Good: The prose and short fiction that accompanies the rules is excellent, evoking a fantastic world and the endless possiblities spread out before the reader. Both short story and examples of play are liberally scattered throughout the text. The core mechanic is pretty simple to grok, and game play can be smooth and enjoyable. Players have alot of options available to them courtesy of a mechanic where you trade attack dice for activating abilities. I expect that the adventures that will be published will be of high quality and entertaining.

The Bad: You need to get at least the Core book, Carreer book, and the Magic book in order to run the game. In many cases, you will need to reference between the books, and often different locations within the individual books, to find all the relevant rules. Additional content and rules can be added either through the supplements or signing up with Patreon, rapidly expanding the content but also expanding the 'where do I find that rule' quandry. The feel of the mechanics is very similar to early editions of DnD, with various fiddly bits and minimum enforcement of 'balance'. There are options that are clearly 'better' than others. Your starting characters are mostly competant and need to grow into being heroes. Generalization is encouraged through the use of an artifical dice cap.

Combat. The 'Fantasy Bestiary' has yet to be published, I beleive this is in part because the dynamics of combat are a bit fuzzy still and the current guidence on monster creation is very close to 'just wing it'. The effectiveness gap between Grades {levels} of characters is exponetial, which means the lower Grade usually gets curb-stomped. The player options that exist are effectively eliminated due to the combat math aiming for a standard 50% to hit rate. Dropping attack dice for a special ability often means a miss.

This sourcebook offers information that will help flesh out your Fantasy campaign using the O.L.D. sourcebook material. I highly reccomend it if you are planning to run a fantasy campaign.