You almost certainly aren't going to see much love for Extra Attack stacking with Thirsting Blade.

As for extra attack stacking with itself, that is something that probably should happen, but just doing that strongly encourages martial multi-classing. What the system really needs is better thought out multi-classing system, and certain class changes to go with it.

For example Extra Attack should probably be a part of a martial power progression tree that is shared by several martials (like spell slots for casters), so instead of stacking extra attack you would be stacking levels on that tree. Another aspect of improved multi-classing would be reworking the vast majority of spells to have up-casting and more efficient up-casting.

Just as an example the martial tree could be something like:
Level 2: Fighting Style
Level 5: Extra Attack
Level 7: Legendary Resistance (Indomitable makes sense for most martials and it should work like Legendary Resistance, and Fighters should have more weapon specific coolness)
Level 11: Extra Attack (2)
Level 13: Fighting Style (or something else that makes sense)
Level 15: Legendary Resistance (2)
Level 17: Extra Attack (3)
Level 20: Legendary Resistance (3)