Just putting my 2 cents here, not sure how difficult it will be to implement.
Could heat be gained through other activities beside combat, like gathering resources?
Say 1000 heat for each herb picked, if i remember right that would mean 380 herbs to reach MP4 and 4200 herbs till MP5 (or 760/8400 with 500 heat/herb/ore/water/etc).
At MP5 heat is not much off an issue, but for solo playing MP3/4 it could help them get to higher levels and have something to do.