Engines & Empires
It’s not something I would do. But if you had a cool adventure and were ready to play i would sit down and play the game and have fun with it. Play it and see how you like it.
Fighters still seem terribly weak, even with these tweaks. Why would you persist in fighter past level 8 or so?
I would say, instead of just 3e core, add in Unearthed Arcana to your list. It has options which answers a few of your "problems" already, and the work was already done.
For example, for level dependent XP it lists as follows
1 - 0
3 - 3000
4 - 6000
5 - 10000
6 - 16000
7 - 24000
8 - 36000
all the way to 20th which requires 2,600,000 XP to level up.
Meanwhile Monsters also off XP in
1 - 300
2 - 600
3 - 900
4 - 1200
5 - 1800
6 - 2400
7 - 3600
8 - 4800
And so on.
it has alternate ways to craft magic items and items (for example Craft Points) and various other alternate rules which could combine nicely into your way of thinking without having to come up with your own ideas.
It doesn't have an alternate feat system (though it does have a return to the original Multi-classing of AD&D in a way, though to really make it work with single classes alongside Gestalt, you would probably need to double the XP each level for the Gestalt rather than keep the same XP tables).
Lots of options in UA.
Of all of these, with exception of 3.Y, none really emulates 3rd edition much.Check out Blood & Treasure
Check out Castles & Crusades
Check out Advanced D&D 3rd edition
Check out 3.Y
I think these will both give you a lot of inspiration, and save you a lot of work.
Even with the core books only, the back half of fighter is a terribly lackluster set of levels.Nah they are fine for what the op is going for, you are propably thinking about tiers and what not, propably not 3.0 and splatbooks allowed. The fact is they will do ok vs monsters with support and again that's obviously what the op is going for