One thing that’s a little unclear from the post - will the alternative levelling rewards also be purchasable with Cubits, or just plots specifically?
In any case, here are some ideas for alternative levelling rewards:
Crafting
This might be unpopular, but you could set a limit on the number of crafting machines a player can build, which can be increased using the reward. The initial limit should be high enough that casual crafters can still get by just fine on a small scale, but serious crafters who want to build mega-factories will need to invest in it.
Gathering, Hunting, Mining
The reward could be the ability to designate a region as part of your ‘hunting grounds’ (not exclusive; any number of players can designate the same region). This would give some quality-of-life benefits while gathering/hunting/mining in that region, like maybe it will tell you how many of a particular resource are left in the region, or maybe even mark the direction to the nearest resource on the compass.
Trading
Similar ability to mark a region to gain a quality-of-life benefit, maybe like telling you what the high/low sale prices are for a resource within that region, or something similar.