Doing a map submission as a collab should ALWAYS be accepted, but it should NEVER be mandated. Pick whomever you will if you want to do a collab.
Like most of these things, the solution is the tried and true formula:
Optional: yes. Forceful: no.
As to seemingly popular concerns, let me answer them individually:
"But I don't have any friends and good mappers would team up with good mappers!!"
...To make awesome maps. So? A collab doesn't necessarily mean the map will be better, in fact more often than not the exact opposite is true and a map's quality becomes inversely proportional to the amount of people working on it. If there is a prize, they'll have to share the prize which is a big deterrent in these things. If there isn't, you don't loose anything anyways, so who cares?
Bottom line, at most you risk putting out more good maps into the community. Oh gee, what a big problem!
"But some people are "PRO"!!"
First of all I despise the expression "pro". It manages to be both meaningless and conceited. You have plenty of time to make the map, you have plenty of time to make your team work, if you choose to have one. If you don't, go solo. Plenty of amazing things have been done solo, and plenty more will be done.
Someone will always be better than you, and someone will always be worse. Toughen up. The objective of these contests should be to inspire the community to put out great maps and have fun in the process. It's not for medals.