Yeah, people have been accusing Fire Emblem Heroes of being a completely anti-F2P game lately in light of the new arena changes. I think that's ridiculous given how generous the game is and how player-friendly the gacha system is (in that it's fairly easy to get any limited unit that you want, but whaling is still encouraged if you want to +10 limited units), but the one place where I can sort of agree with that is in the handling of the starter units. The weapon refines are nice for as maligned as they are, but the fact that the starts have been kept strictly un-mergeable is pretty unfortunate.
Given the starter units' status as unsellable locked units traditional merges don't really make sense, but I'd love to see some kind of process for upgrading them. A perfect world would give us some kind of storefront to spend arena medals and badges that sells +1s to starter units, easier access to some rare abilities (it does't even need to be the super premium ones, just more variety) and maybe feathers and orbs at a steep exchange rate.