XP going down by doing the same thing again. 1st brahmin 80xp 2nd brahmin 70 xp 3rd brahmin 60xp after 8 : 10 xp
Double the number of xp you need to do next level : lvl 2 1000 xp lvl 3 2000 xp lvl 4 4000 xp lvl5 8000 xp
Or skip the level cap : So the character get hard to level up, and you make less characters.
Get a special random (not chosen) bonus, every 500 000 xp point. (so you will still want to get xp with the same character. (could be a lvl3 weapon, a perk, a skill point, more hp. anyway, you don't choose it)
Make new upgrade that would prevent abuse of lvl1 char :
5 magnetic personality perks, so your upgraded leader will be better than lvl1 leader. (8 mercenaries instead of 4) (same thing exist with AP, HP, skill point, carry weight, why not charisma ?)
Need to increase science and outdoorsman to make a tent. (let's say 60%) So, you character has to level up in order to build a tent. The tent would also disapear, if the one who built it doesn't go in regularly. (and not just someone)
Need to be level 10 to be a faction member, whatever if the faction is player-driven or npc-driven. (so these alt have to be levelled, to take advantage of the faction)
Your armor and weapons would have a far lower deteration rate if you have science and repair (passive skill). Or, in the opposite, if you haven't, your weapon could go beyond repair, at any moment. (at every critical miss, like a jinxed) With your crafter, you're not a better fighter, but you are sure to keep your weapon more than 10 minutes.
With a low charisma, you're more likely to be targeted by npc (that can choose between you and other ennemies). And you have more chance to get collateral damage. (ooops, or zone damage) Yeah, your friend can love you, but, not much. They don't care if they shoot you.
With an high charisma, you can hit some npc, having a chance of not getting hit back.
Others character requirements could be made in order to prevent abuse of lvl 1 alts.