I disagree.
early heros already get an advantage on late heroes as they come early.... and are thus much more useful than late heroes..
I've always had lucian, bambur or gilden silveric have much more impact on my defense or attack strategies (at the very least by allowing me to spend less in units and more in techs/ development) than magnadine or the war machine or sephner.
in conclusion, late heroes are much stronger than early heroes, but they are less game-changing... and are not "so" much stronger as by the time they arrive, you have much units with more than 100xp, and the "early" heros have more than 100xp... and can thus often take the "newly born late hero" out of combat very quickly.
giving a further bonus to early heros would advantage them too much against late heroes, which are already weak (as they are not as game-changer as early heroes)
lastly, you have to consider that the "heroic attack/ heroic defense" promotions ARE exactly the thing that was implemented to allow early to heroes to have an impact mid-late game... (they get +1 str after level 7 and +1str again after level 9 (but you have to spend xp for that .. and caster heroes get the "twincast promotion" that double their caster's effectiveness)
you need at least level 10units.. so 101xp or so to get combat V and HA2 and HD2... which early heros can get... and. late heroes cannot often get this much xp in time.
for Bambur, with this, he goes up to 7str... which is stronger than a champion (and he can cast a spell, and move 2mvt in hills) !!! AND champions are late-game units, or at least units you still use late game. Lucian goes to 6str.. as a champion but with the 10%vs city of axes, Rantine too (6str).. + his spell and +25%vs melee units
Saverous goes up to ... 9 str !!! +(metal weapons).. as strong as an immortal (but without the immortal and with less movement, but immune to poison and many spells)
gilden silveric is strong too 5/6 ==> 7/8 (as much as a longbow with iron weapons.. without needing to find Iron)
Maybe you can say that Lucian is lacking a bit... as he gets nothing on top of champs... but you get him for FREE !!!
the only one lacking is Barnaxus.. but well it's a golem (what is more stupid is that Luirchips don't get any "hero" with the hero promotion, nor hero that can be resurrected using Life III (while they can get archmages))
I think you are underestimating early heroes a lot, and their progression curve.
In fact, I think that the one who would need more love are late heroes.. they should get much more strenght or special effects than what they have at the moment !