Opinion seems to favor the Fantasy Heartbreaker, so the other idea gets back burnered for the moment. So with that in mind, I need to think a little but about first principles, and that in turn means thinking about influences that I'll be keeping in mind.
The first is, of coursed, D&D. But that's a wide swath of material, and deserves some drilling down. Now, personally, I have fond memories of red book basic D&D for its readability and accessibility. I have a great deal of respect for the OSR's desire for simplicity, but I have always differed from them in that I feel simplicity has been more successfully achieved by games which are not D&D and therefor not part of their discussion. So, while I have an eye towards the spirit of slim, simple books, the tools I may use to get there may be quite different.
This may be harsh to say, but I can't think of anything I'd steal from first edition Advanced Dungeons and Dragons. There are maybe a few ideas which had strong underpinnings but poor expression but there are other places I'll turn to for how those ideas eventually got expressed. I mean, I loved these books, and I'll try to keep the wahoo sensibilities of the DMG in my back pocket, but it's hard to separate what's truly useful from what is compelling due to the time and place I was when I read them.
That said, I want even less from 2e. I think it was an improvement on 1e in almost every way, and it was solidly playable, but it was also very bland. However, that blandness also meant it ended up supporting some of the best settings I've ever seen in RPGs, with Planescape, Birthright and Red Steel jumping right to top of mind. The mechanics may not have left much of a mark, but the settings absolutely did. Not sure what, if anything, I'll end up dong with them in a heartbreaker, but they're definitely on the table.
3e, and all the things born from it up to and including Pathfinder, Fantasycraft and other current incarnations, did a lot of things right. Most notably, the level of character customization was, when good, magnificent. If you look through older 1e and 2e products that represented fiction, like Conan or Lankhmar, the characters almost always broke the multiclass rules to try to capture the correct sense of them. In 3e, making those characters was now in bounds. The addition of Feats was also noteworthy and also emblematic of everything within the d20 sphere - potentially very useful, but potentially very cumbersome. There's so much stuff here that there's no useful way to narrow it down, but it would be foolish to go forward without it on hand. Almost anything you can think of in fantasy has at least been tried (not necessarily with any success) in d20, and it's always useful to see earlier efforts.
In contrast, 4e is much more focused, and that's nicely reflective of what it brings to the table. More structure, more clarity of action, and a much stronger metagame. It put different characters on better footing (explicitly pulling non-spellcasters out of the gutter and onto the table). I definitely want to draw heavily from here, but there are things I want to explicitly avoid, primary among them being the decoupling of color and mechanics. It is important to me that game effects be described in game terms, not created in game terms. Really, in general, 4e is a smooth, well oiled machine for miniature combat, so taking out the parts i need to do something else with it will probably make for a lot of smoke and noise.
Beyond D&D, I first turn to Rolemaster. The brutality of the combat, the diversity of the characters supported and the range and potential of the magic system are all huge advantages. The one downside is bookkeeping - RM had a lot of it, and that doesn't fly well. Still, the ideas are mineable, and things like Run Out The Guns and 3E D&D have demonstrated that many Rolemaster ideas can be streamlined.
Earthdawn is also worth keeping in mind as a game that hangs together almost as well as 4e, but does it through setting elements. Fantastic setting design, the best magic item system can think of, coupled with the conceit that EVERYONE is using magic, just not necessarily to cast spells. In many ways, Earthdawn and 4E were made for each other, but those two trains will likely never meet.
Am I a bad gamer because I don't care much about Runequest? Or Harn? I'm ok if I am.
Another obvious influence, the one that got me thinking down this path in the first place, is Green Ronin's Dragon Age. It's a simple, elegant system that showcases many of the benefits of random character creation and has a really fun, expandable mechanic with its dragon die. By and large, I only wish there was more of this game to steal from, because man, it's awesome.
Exalted has a bit of a back of the mind presence, but it's so much its own thing that it's hard to even view it within this cloud. Still, 1st ed Dragon Blooded was so mind-bendingly good that I suspect it will be hard to totally shake myself of its shadow. Similarly, things like Everway, Amber or Pendragon are so much their own thing that it's hard to look at too closely, though they'll never be too far out of mind.
The big-setting games - Talislanta and Empire of the Petal Throne specifically - are in a similar boat. They are so much the expressions of a vision that they are brilliant to explore, but hard to use without feeling like a mooch.
I dig Warhammer Fantasy RPG, but I'm nto sure if there's anything I'd actually take from it except perhaps the Lifepath system, and in practice, Burning Wheel has already taken and improved that particular technology. Now, Burning Wheel is definitely something to steal parts from - the game as a whole has never quite been my bag, but it's made out of the highest quality parts, many of which deserve to be borrowed.
Obviously, there are games beyond the fantasy sphere I'll draw from, but since the starting point of a Fantasy Heartbreaker is it's D&Dness, I wanted to start from that base. I'll pull in other games later as needs arise. But that said, what baseline fantasy have I overlooked in my thinking?
1 - Erol Otus Cover, though the Choose Your Own Adventure part of the one with the Elmore cover was pretty sweet.
2 - Old School Revival, which changed its name from Old School Renaissance for what may be slightly silly reasons. It's a small but talkative movement of players who thrive on old versions of D&D.
3 - Like the Weapon vs. Armor table. In theory, this was an interesting way to differentiate weapons, in practice it was too complicated to use, and did not actually work in conjunction with the way the armor class system actually worked, since the actual number of your AC did not necessarily correlate to any type of armor, especially for monsters. Yes, it was base AC, but that was just extra fiddliness. It's easy to argue that ti made sense, but the simple reality is it saw little use, and even the streamlined version of it in 2e was left by the wayside. Rolemaster did it well by baking it into the system, but it's safe to say the RM comabat system was awash in tradeoffs.
4 - In previous editions, Fireball was a sphere of fire that exploded to a particular size. That's what it did. Rules existed to reflect what happens when a ball of fire explodes (though they were not necessarily consistent in this - each spell was it's own little packet of rules, and that could cause some muddle). In 4e, a fireball does fire damage to a specific size area on the grid. It matters not at all whether this is an exploding ball of fire, a rain of fire, fire leaping from the ground, or heat vision so long as the mechanical effect is the same. This works fantastically well within its sphere, but becomes a problem when you want to actually bow something up by, say, throwing a fireball into an enclosed space. In an older edition, it might blow a door off or the like, but in the current edition, it does no such thing. And fireball is one of the EASY powers to model. Others are almost impossible to visualize.
5 - The tradeoff is that the new edition allows for easy reskinning of powers, monsters and everything else. If you turn the fireball into a rain of fire in old editions, it changed how people thought about using the power. In 4e, it just changes how they visualize it. This is powerful and useful.