Just had an odd idea - what if you played for a territorial victory? Each unit has a minimum territory it must own or occupy. For pawns, 1 square, the one they currently occupy, is sufficient. Knights require 2 squares, the one they occupy and another empty square unclaimed by any other piece. Bishops require 3, the one they occupy and 2 other unclaimed squares. Rooks require 4. Queens require 5. Kings require 9. 

Occupation of a square is obvious, if you occupy it, you own it. But what about empty squares? Who owns empties? This is determined first by control - if both sides can capture on that square, neither owns it. If only one player attacks that square, then that player owns it. If neither player attacks the square, but it is behind the pawn/piece lines of 1 player, that player owns it. 

Victory is by checkmate or by being the first player to own enough squares to satisfy that player's current army. Squares do not have to be contiguous for individual pieces, but the entire kingdom's squares must directly connect to every other owned square. The kingdom cannot be separated into disconnected pieces voluntarily, although an enemy attack could force a disconnection. If this occurs, the game cannot be won by the disconnected side other than by checkmate. 

So the smaller your army, the easier your victory conditions. The values I've given seem, off the top of my head, to make for a playable game, although adjustments could be made. For instance, you could reduce the king's required squares to 6, to possibly speed up the game, or make it more playable, or to handicap 1 player. 

Just a thought, fueled by insomnia and who knows what else. Enjoy.