D&D 5E Warlock and adding more power to the summoned creatures

[Athinar]

I was thinking about how weak our Warlocks are in combat, can they summon more powerful aid than say the imp or sprite or even cast a spell to enhance the creatures, say by adding Hit Dice, Hit Points, etc... for a short duration spell, say 1 minute

Something like how WOW does it with Warlocks and the summoned aid