I dunno, its always made sense in my head that you start with a Demigod. If you want to emphasize his general abilities, then max up the abilities in those areas and buy items that will give him the edge in that area. Likewise if you want to emphasize an assassin type, you max up those abilities first, and pick items accordingly. Take for instance Furion in the game of Dota. He has the following abilities:
1. Forces of Nature - Converts up to 5 treants from trees.
2. Teleportation - teleport anywhere on the map (30 sec cooldown when maxxed).
3. Sprout - sprout a ring of trees around the target trapping him in.
4. Wrath of nature - Summons damaging energy to swath around the map and damage random enemies. Each enemy hit beyond the first adds 7% damage. Number of bounces increases per level.
So, when you start if you max out forces of nature, buy aura items, you become an early pusher. Your strength is in numbers and your focuses on pushing down towers and the enemies base. In earlier versions of Dota (before backdooring became frowned on or even banned), furion could teleport into the enemy base, usually hiding behind trees in the base) until it was empty, sprout 5 treants, hit refresher (item), spourt 5 more, and take down a few key structures, then tele out. His focus was not on killing other heroes (assassin), but to push with his treants (general).
However, you could opt not till level forces of nature till later. Max sprout and teleportation. Save up and buy high damage items (desolator was a favorite for Furion before it was nerfed). Buy some wards (observers) and place them around the map. Whenever an enemy player is damaged, teleport to him, trap him, and finish him off. Or just initiate the attack, teleport from behind, ambush, attack, attack.
The order of abilities you choose to level and the items you buy decided his playstyle. You could try to level all the abilities evenly, but honestly, you contribute less to the team by trying to be a mediocre jack of all trades. This was just an example. I'm not sure how Demigod will work, but I always imagined (since they said the lines may be blurred between assassin and general) that you can specialize your demigod into one role or the other. Maybe some Demigods will be Assassin only, and some General only, and some somewhere in between. Who knows.