Combat in Dungeons and Dragons is pretty good at representing small pitched battles in dark caves, what I feel has always been a weakness of any D&D system is running battles between the party and large powerful creatures, also how these creatures are presented in general has been a weak spot in the system (in my humble opinion). I am planning on writting my own personal re-imagining of BX for personal use. I've done a bit of it already and laid some groundwork. This weakness regarding powerful creatures is one of the things I hope to address, and I have quite a bit to say about it, and I will be returning to this subject many times as it develops.
One problem I have is that fighting say a dragon is not really all that different from fighting a goblin, sure one has more special rules and better stats- but fundamentally the game treats them the same. I can see why this came to be from a design standpoint, they created a system that everything can fit into, that's not bad in itself. I argue though that creatures like Giants, Dragons and Purple Worms should be treated differently, that fighting a Brigand and fighting a Roc are 100% fundamenatlly different situations and I believe they should be treated that way in the rules.
Fighting a legendary creature should be a unigue and complex challenge, not an HP grindfest with some extra rules thrown in. I am looking at games such as 'Shadow of the Colossus' 'God of War' and the recent 'Castlevania Lords of Shadow', as well as various fantasy and adventure films and popular fantasy literature to draw inspiration for the rules I'm writing. More on this later.